Switching Permission

I am a little lost on when it comes to switching perm from 1 to 5 or 2 to 3 etc. Is there a small solution so players can auto change from one to another

Example they have access to perm 1. They use in-game rewards and purchase perm 5. They have multiple perms all ready defaulting back perm 1. Solution to adding to new perm and removing from old or using highest they have access to without removing