100% optional. For most people, the time that could be spent going back to fix up old pages is better spent just continuing to make new stuff instead. Mostly just because we all have limited time and so usually it's one or the other: make new stuff or redo old.
Some people even get stuck in a "redo loop" where they never get anywhere with their story because every time they finish a chapter they look back at the previous one and think, "gee, my art got so much better now, I should go back and update some of the first pages with the better art!" but then they finish that and their art is better again so it's like "hmm... well maybe 1 more redo wont hurt..." and so on. I was one of these people, for like 10 years middle school through college lmao.
There are times updates can be good though. After you're done with a story, go nuts if you want to. Or especially if you're wanting to print or sell your comic, after finishing it can be a good idea to go back and at least touch things up even if you don't redo a bunch of pages completely.
But generally speaking just keep moving forward!