I would decide whether to do it or not based on both the planned length of your story (and the current place you're at) and your own experience.
Chances are if you don't have much experience writing (or writing web novels specifically) and botched something up once, it might occur again so you'd have to rewrite several times. In that case, it might be easier to first finish the story and then do one big rewrite where you can make use of all the skills you've gained so far. This also has the advantage that you're not creating the conundrum between new version and old version where some readers started with the old one and then have to switch over to what is the new one (or wait until it catches up) where things might go (slightly) different if you change the plot. If we're just talking about a revision of writing style, this wouldn't be an issue though and you could basically just go for it and slowly work through the old chapters to make them flow better.
As for the length of the story: If you have, let's say, 200k words planned and are pondering whether to revise at 150k, I'd say again to finish first and then rewrite because those 50k will likely not take that much time (my perspective might be screwed there though, that's about half a month's time for me so really not long). If you have planned for a million words or so and are 150k in, then the story overall will still be running for a much longer time so having a beginning that reads better might be very beneficial for gaining new readers so I'd say to go for the revision immediately.
Personally, I am currently in a somewhat similar situation: My main series was planned to last for about 1 million words but we've long surpassed that and there are, like, four more volumes to go right now so I'd say it's safe to assume we might end up with 2 million or so instead. It was my first web novel (but I did previously finish, like, 10 or so traditionally written novels so I wasn't completely new to writing), as well as my first English project, so the beginning wasn't great to read either.
Now, I felt I couldn't just stop writing where I originally was and continue on the revision since I didn't want readers to wait for so long (let's be honest, at least half of them would drop the novel if I did that). So, basically, I decided to make it two versions that are running simultaneously: I am continuing to write the original version at my usual pace that I'm uploading on three sites but also editing/revising/rewriting the old volumes and uploading them as a new version on another two sites at the same pace. When I finish up until the end of a volume or a certain milestone plot-wise, I exchange the old version for the new one on two of those sites.
This way, readers can decide which version they want to read and can even jump from the new version to the old version at some designated points if they'd like to because they don't want to wait until we catch up with the old version.
So far, this approach has worked quite well for me save for the fact that I needed to take some breaks in the middle of the revision recently because of real-life issues. It works for me but I'm not so sure if it would work for others. The only question is whether you're able to keep both versions separated in your head. As for uploading, you could also put both on the same site as long as one is clearly labeled as the revised version and then link the two of them in an author's note or something or explain in the blurb.
