We've had good luck with DTD, MK guest relations and DHS guest relations. I like doing it at a guest relations location working on the idea that there are lots of people in there if one doesn't know they can easily find out from someone else (ticket booths you just seem to have one person so it would be more hit/ miss). DO go in knowing how much you should pay to upgrade - if the CM quotes a different amount be prepared to ask for a supervisor or go to another location.