When they purchase a username change, it's completed immediately provided they properly fill out the form asking them for their new username.
If they purchased it as a gift for someone else, it will be the other person's username that changed.
Purchased user titles I believe won't reset, no, as they'll show up as "admin set" so they won't be automatically overridden. Same conditions as new user name, though.
Disabling / Removing it won't undo those modifications, but it will undo everything else, like permission overrides and colour overrides.