You should be able to do it at any guest services but people have had varying degrees of success, it all depends if the cast members knows how to work the system. Once you get one that knows how to do it then its down to price, seems you can get different prices from different places. You need to find one you are happy with.
I upgraded my 14 day ultimate tickets on the 18th May, to a standard annual pass cost to upgrade
Adult $216
Child $205
The CM had to find out the cost we paid for our original tickets so they could give me the upgrade price (booked with the free dining) and our tickets were buy 7 days get 7 free. so the above price was for both the ultimate and the premium uk tickets.
As we are going back Aug and April this was worth it, then if you want to add waterparks you can do this at anytime and the cost done pro rota, however if i had used the waterpark/disney quest option on m ultimate would have had to ugrade to the premier pass which i think is about $150 more.