I've only ever posted one story on this site, and it was a rewrite of one I'd posted somewhere else a few years prior. I took large chunks out that I thought were unneccessary, or redundant, added parts in for clarity. Fixed a bunch of spelling and grammar errors that drove me nuts when I saw the original! But my motivation was because I wasn't fully satisfied with the original, and I thought I could make it better. The original I wrote over a 6 year period, posting slowly along the way. I never made an outline, I just wrote what came to mind. And that, naturally, led me to write myself in a corner many times. This time I was able to map it all out, and make things flow better. I did want to rewrite one character, but I couldn't because doing so would have meant the entire story would need major structural changes.
If you do decide to rewrite, I would suggest doing it all before posting. I did all of the changes before. But then once I put the first two chapters up, I don't know why, I decided to basically rewrite my entire rewrite -- and made major changes. I wouldn't recommend doing that!