One of my players is a Warlock 3/Fighter 2 multiclass Half-Elf. We haven't played for a while as we have all been either busy or away on vacation and so she decided to take the opportunity to reconsider her "in-game" life choices. She asked me if I could let her "drop" the two fighter levels and add them to the Warlock instead because she decided she doesn't want to be much of a fighter.
I am worried that by allowing her to do it, it could cause problems in the campaign since other players might start wanting to change their characters (They are all single-class so they might want to become multiclass with their current levels) and they would have a justifiable reason. Allowing them to do it right now would create problems story-wise because the campaign is sand-box I have based entire quests on their backgrounds, races and classes. This character's "quest" is based more on the "Warlock" side rather than the fighter and so it won't be a problem at all. But other players will cause issues such as having to delete entire missions along with their side quests and I would need to write brand new ones and I don't have that much time as of right now so the campaign would become unplayable.
My question is; What's the best way to allow her to do it without causing the aforementioned issue?
Note: It's my first time DMing.