In terms of redoing entire pages, I think it's a matter of weighing the pros and cons, and oftentimes it's just not worth the hassle. But if you're taking your comic to print, for instance, it's a good idea to revisit early pages to ensure a consistent look.
Redraws can spiral out of control when you don't have a specific goal for re-doing old pages. If the goal is to catch the artwork up with the newest pages, it's easy to get trapped in a never-ending loop when your old pages start looking better than your newest pages, and then you have to fix those, and so on...
You also have to consider if you can re-do old pages while not falling behind on new ones, which is hard for a lot of people and might lead to burnout.
But if you have a specific goal in mind, or just have minor touch-ups to the old pages, then it's probably not that bad to go back and fix them. Like I mentioned earlier, I'm going to re-do all the lettering for ch 1 of my comic, but I know that won't take as long as re-doing the artwork. If I do make changes to the artwork, it's just going to be minor touch-ups. But that's mostly because I'm planning my comic for print-- otherwise I might be inclined to just leave it.
At the end of the day, though, it's up to the artist and what they think they can handle!