This is sort of already a feature coming to iOS 14, though it's really for switching an existing account to an Apple ID login, there's nothing technically stopping a developer from using it to switch a user from FB login connected account too.

If an app provides 'login with <something>' the app now has to support 'login with apple' as well. They are not activly blocking apps that violate this rule, but soon they will.

In our app we've already added a "Link your Apple ID" option to add Sign In With Apple to an existing account, it's easy enough to implement without any official support from Apple.