I try to not do major modifications of content that is already published but It's quite common for me to do small tweaks. In general I fix spelling/grammar mistakes and, since I'm using the scrolling format (which I'm not used to), also readjust the panels (their position or the spacing between them).
Even if I try to avoid it, it's also common to modify some text for the sake of clarity or change small details in the drawing.
For instance the last change I have done in an already published page:
When I do this changes I'm more satisfied with the final result but at the same time it annoys me the fact that I spend on them some time that I could have used to draw new content.
My advice, that I'm trying to apply to myself, would be to plan better in the initial stages to avoid wasting time later fixing things. Moreover, if you are working with a tight schedule it might be better to forget about the published content, assume it's good enough, and focus on the new one. If you are drawing for fun, and don't care about publishing sooner or later, just do whatever you want, do as many changes as you need until you are satisfied with the result.