If a passenger self-upgrades and refuses to leave but is otherwise "non-disruptive" then I would have thought that the most likely sequence of events would be to continue with the flight, notify police at arrival airport that passenger has been disruptive and non-compliant and cancel the passenger's return ticket (if applicable) and in any event place them on a no fly list. If a decision was taken to divert then I imagine the passenger must have been abusive and/or using threatening behaviour.