It's like straightening a wire.
Ret-conning all the old parts to match up with the new parts, while trying to make the character arc as smooth, believable, and heartfelt as possible. Sculpting all the scenes through a nice logical cause-and-effect progression, while aligning them with the larger motivations of the characters.
It's a difficult, infuriating process--a tiny counter-bend here that throws off everything at the far end. Small kinks that will never be completely straight. Constant stepping back to assure yourself that yes, it's still going in more or less a complete line.
Occasional self-assurance that most of your readers won't notice or mind that slight bend in the conduit.
Worrying about the ones that will.