Generally speaking, the ease of making changes to the game, its narrative/characters/content is inversely proportional to how close that content is to release. The closer the patch/DLC/update is to release, the more locked-down and set-in-stone it is. This is because these kind of changes will often affect other systems, narratives, characters, systems, etc. and that kind of propagation through the system gets more expensive the more changes need to be made later on. I'll use my current project as my example.
My current project is getting close to launch. We already have the first several seasons of content for this game scheduled - we plan on adding several significant boss fights and content updates over the course of the next year. The first new boss scheduled to go live in the first post-launch update is the closest to being done. That boss already has all of its mechanics locked down, all of the art done, all of the voice over recorded, all of the features designed, and so on. At this point, we're just tuning the fight because it's so close to release that we don't want to risk breaking anything. No matter what the community reception is for this particular character in the narrative, they're getting this fight because we've already locked in those plans months before launch and it's too late to change them now.
The further into the future we go, however, the more malleable things become. A good rule of thumb is to assume we on the dev team are working two or three patches ahead of whatever you're currently seeing in game. Content patches are often quarterly releases, which means that the thing I am working on today probably won't be seen by players for at least 3-6 months down the line. If supporting character Neelo turns into a huge fan favorite, we can pivot to add more content to feature Neelo - and sometimes we do! However, even if Neelo becomes super popular right at launch, that likely means the fastest we will be able to add more Neelo to the content updates (if we didn't plan to do so before) is in around six months post-launch. The patches before that are already in the process of being locked down and getting ready for certification and launch, it's too late to change them.
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