Let's be honest: the first few chapters of your fanfic are horrible. You know this! These days you can look back on the awful, awful decisions you made when you were young and foolish--maybe a year ago now, maybe five--and cringe. How did you ever think it would be a good idea to give your protagonist a shiny Manaphy starter? Why did you set up that inane bit of lore about how your main character is able to talk to pokémon because of the special bracelet their dead mother gave them? How did it take you that long to learn how to spell "Gyarados?"
You don't know, but the problem, now that you're up to chapter thirty-whatever, is that those ridiculous newbie mistakes haven't gone away. They're still out there for the whole internet to see and laugh at. Worse, they're the first impression any new readers are going to get of your story. They'll laugh and back button out of there so hard their trackpad catches fire and tell all their friends about this hilarious badfic they found that has all these reviews, God, how are people such morons, and everyone will know what a terrible hack you are. Maybe you'll even get some self-righteous critic strutting into your review section to let you know just how disappointed they are that you ever thought those early chapters were a good idea. The embarrassment just doesn't stop.
Fortunately, you have one recourse: rewrite those chapters. Heck, forget just those chapters: you can rewrite the whole story! Not only can you fix all the wince-worthy stuff in the early going, you can patch that unsightly plot hole in chapter sixteen. You can even make some real structural changes, alter characterization, or take the plot down a completely different course now that you're not locked into poor decisions you made early on.
This is certainly all true, and you can start in on a rewrite. But me? I think you shouldn't, at least not if you hope to ever actually finish your fanfic.
First, let me get this out of the way: I'm not saying that going back and editing some of your story is a bad idea. Feel free to spruce up the first couple chapters. Retcon out that prophecy all you like. And please do find-replace "Gyrados" for me. What I'm talking about here is complete, start-from-scratch, do-it-over rewrites. These and quick revisions are very different beasts.
When you start over from scratch, you drop all obligation to continue working on the present iteration of your story. You'll be devoting all your time and energy to going back to the beginning rather than pushing through to the end of your current draft. On the other hand, if you just want to nip back and rewrite chapter one, you probably still intend to have chapter forty-whatever out in whatever you might consider a reasonable amount of time. Even doing a pattern of rewrite one, post one keeps you on track to cross the finish line at some point. It's the suspension of new work in favor of retreading old ground that's dangerous, not the act of polishing up your old prose in and of itself.
And why is doing a big rewrite such a dangerous thing? How does it prevent you from finishing? The most obvious, practical reason is that serious rewrites represent a huge investment of time and effort. Most long chapterfics take years to reach completion even if they're being written straight through. Let's say it'll take three years for you to plow through your epic, for example. If you decide to rewrite somewhere around the middle, you'll likely be looking at another year and a half of re-doing the first part--ignoring that rewrites are often slower than initial efforts, for reasons I'll go into later--plus the final 1.5 years to make it to the end. All said and done, you're looking at almost half a decade wrestling with your story. That's quite a marathon! The longer the writing process stretches, the more difficult it is to sustain enough motivation to carry the story forward, ignore the temptation to get wrapped up in other projects, and keep your interest in the tale alive.
But I think it's more than logistics that cause people difficulty with rewrites--and I do think they tend to be harder than writing straight through for the same amount of time. Rather, I think the issue with rewrites has more to do with the underlying cause of a lot of them: perfectionism. Most people aren't concerned with going back and doing a rewrite when they're racing through their current chapters. When the work's exciting, when the prose is really flowing, you don't have the desire to stop and look back. Why would you? What's up ahead is way more exciting, and so is the process of getting there.
On the other hand, when the way forward is murky, when you're struggling with a difficult chapter or a lack of motivation or any other of the many obstacles that trip writers up in the course of their work, that's when it's easy to consider turning back. You really want a break from your struggles with the story, and compared to the rocky, unfamiliar ground that lies ahead, those first few chapters are starting to look cozily familiar. You always wanted to fix that awful early dialogue, anyway, and now that you understand Natalie's personality better you can make her introduction really shine. If you hadn't set up that ridiculous situation midway through chapter fifteen, it would be a whole lot easier to resolve the dilemma you're facing now. And plus, doing a rewrite will be easy! You've already written this before, right? You know how it's supposed to go. And it'll come out so much better this time...
This is rewriting as escape, one of the most seductive outs offered on the long climb towards finishing a story. Finishing a chapterfic, even a relatively short one, is no mean feat. No matter how bright-eyed you are when you start it off, you'll hit doldrums somewhere along the way, and going back to rewriting on relatively familiar ground will look increasingly appealing. What's more sinister, rewriting is a virtuous-looking out: not only do you get out of a bind, but you improve the story in the process, do what everyone says you ought to: reflect on the criticism you've received, the mistakes you've made, and then go out and do something better. You get to feel like you're accomplishing something, when in fact you're stuck on a treadmill of rehashing the same ideas over and over.
Yes, the urge to rewrite can be very tempting. If you've made any substantial progress on your story, though, you already know that moving forward requires resisting temptation. The desire to drop everything and start a rewrite can be every bit as destructive as the desire to spend an evening on Netflix because it's an urge that often comes from a place of fear, and if you give in to that fear, your story is never going to get finished. It's the fear that what you're writing isn't good enough, or that what you've written isn't good enough, that people are going to hate your next chapter, hate the entire story, hate you and call you out on being a terrible writer.
There's nothing wrong with being worried about those sorts of things. It is scary to put your work out there for other people to read. It's admirable to "have standards" (a common perfectionist claim) and want your work to be the best it can be. Making edits and revisions is hardly an evil practice; generally speaking, it's a good habit to get into. But as with all good things, it's possible to take it too far--sometimes easy to take it too far. Doing a big story-wide revamp is usually going too far, and it's often less about actually making the story better than trying to satisfy your ego, to convince yourself that you're not just a writer, but a good writer.
This is where perfectionism comes into play. It carries with it the belief that, at some point, your work will be good enough, and that if you just do enough editing, you'll eventually reach it. Once you're satisfied with the quality of your prose, once you're feeling nice and comfortable and confident that you've done the best job possible, then you can go ahead and post your work.
The problem, of course, is that you're never going to be satisfied, especially not with your earlier work, e.g. the beginning of a long chapterfic. You'll always be able to go over it later and identify elements that don't jive. This is only natural, as your writing will tend to improve with practice, and if nothing else your tastes and interests will evolve over time. Thus, if you throw yourself into a rewrite, by the time the new version catches up to the original, the early chapters will probably look just as bad to you as the messy chapters that inspired the rewrite in the first place. And then what are you going to do? Start over again?
Posting a story is always going to involve some level of discomfort. You can never be certain how something will be received, and not knowing is scary. It's rare that you'll be able to put up a chapter that you feel is truly the best it could absolutely be, one that you're sure has no flaws. To move forward as a writer, you have to learn to make peace with that uncertainty, to embrace the fact that you're going to screw up sometimes--and that no amount of agonizing beforehand can ever prevent that. You aren't perfect, neither is your writing, and it's simply impossible to revise it to perfection. If you give in to the temptation to turn back, you're not only feeding a bad habit in letting your fear get in the way of your progress, but you're also setting yourself a stiff challenge. Revisions are no easier than producing new material, assuming they're more substantial than fixing some comma fail. If anything, they're probably more difficult: you feel additional pressure to produce something really good to justify the revision, and your expectations for your writing are higher now than when you went through these chapters the first time. And, of course, you're going to re-encounter the spells of motivation loss, uncertainty, and general writing blues that got you discouraged in the first place.
To some extent, I think people recognize this. There can be an element of self-sabotage in gearing up for a big rewrite. Some writers are simply putting off a tricky bit of story in the hopes that the next time around they'll be better prepared to handle it. Some may have a particularly exciting ending planned and be looking forward to writing it--but be as afraid of screwing it up as they are excited to share it. In these cases, there can be a psychological benefit to not finishing. As long as the material stays in your head, you can imagine it as perfect, can savor the anticipation and the expected reaction to the work. Indeed, the unwritten future is downright useful: you can pretend whatever you want about it. Readers questioning your mystery? Just wait and see, there's an explanation coming up! Critic doesn't like that character arc? Just wait and see! They get a lot of development in future chapters!
If you actually write to the end, there will eventually come a point where there are no future chapters, and readers will be able to judge only what made it onto the page, not the awesome vision in your imagination. You, too, will no longer be able to enjoy the unblemished excitement of what might be, in the face of what actually is. There's a great deal of satisfaction in being finished, of course, and real joy in being able to share the last piece of a project that you've poured so much effort into. But there's comfort in not finishing, too, in being perpetually in progress. Your story is familiar now, as is the process of working on it; it's been in your life for several years, maybe. You lose that upon finishing and gain an unwelcome visitor into the bargain, the nagging little question that haunts all writers who've just put the last nail in a story's coffin: "So, what next?"
In the end, while the reasons people cite for starting revamps are numerous, many ultimately come back to fear. Rewriting is a move that helps mitigate that fear, but it's also one that threatens to trap you in a cycle of retreading old ground. Every long-term creative endeavor requires courage to complete; it requires taking risks, pushing past dissatisfaction with your work, and forging ahead even when the desire to quit or go back is overwhelming. In the end, you'll need to make peace with any discomfort you have with your work and move ahead anyway. Remember that it's your ego that wants you to look good all the time, that wants you to do things perfectly, that aches for the recognition that you are not just a writer, but a good writer. The ego can never be satisfied. If perfect is the enemy of the good, perfectionism is the enemy of the good writer.
So: no revamps. What are you to do, then? If you want to spiff up your story but accept that a revamp is a bad idea and aren't satisfied by rolling revisions, I suggest doing a revision after you've published the full first version of your fic. If you're still interested in the story idea and feel compelled to take another swing at it, that's great! You've already accomplished the goal of getting through a complete fanfic; even if you do end up dropping your revision, you'll still have grown from that experience and be more prepared to see projects through in the future. The "post-posting rewrite" also has the advantage that you'll be able to consider your story as a whole when deciding how to change things around. Often the most important themes of your story only become obvious once you've reached the end, and going back through to bring them to the fore is a luxury you don't have when you're setting out to rewrite after getting only part of the way in.
What's probably most interesting to me is how universal the problem of rewrite fatigue is across creative disciplines. For example, the title of this essay is taken from an article discussing why it's a bad idea to do rewrites in software. Back in the days when fansites were a big part of the Pokémon fandom, it was well-recognized that a site "hiatus," where the webmaster would take everything offline while they redid their pages, was a good sign the site would never come back. I'm sure you've followed a fangame, comic, or other piece of creative work that went dark forever after the creator announced that they were starting a revamp. And in over a decade of reading fanfic, I've never seen a chapterfic that underwent a "from square one" revision that actually reached completion. Thus, whether or not you agree with me about why rewrites tend to kill fanfics, I think the anecdata make it clear that they do. It's not that big rewrites can't be done, but only that in practice they rarely work out well. You may be convinced you can beat the odds, and perhaps you can, I think it's worth at least taking a step back and considering exactly why you're so fired up about doing a rewrite. More often than not, I believe the correct decision is going to be to set those reasons aside, take a deep breath, and plunge ahead anyway.