When joining a campaign via the invite link you have the option to join with an existing character that isn't already in a campaign
If a character is in a campaign, they'll have a red Leave option below their character card when viewing them in the campaign and a red Leave Campaign option when viewing them in My Characters
I want to be able to pull some of my old characters into a campaign and then remove them without deleting them.
You can already do this
Find my D&D Beyond articles here