They will not/cannot do this over the phone. They must scan your tickets. We have park hopper plus with 5 days left on them. To convert them, we must go to the ticket window at one of the parks, have them scanned, and the price converted into today's dollars so we know how much we need to spend to upgrade them.
It's annoying that they can't/won't take the ticket number over the phone so that I can have an idea and be able to calculate which tickets would be best for us........