Well, this might be tough since they are one day tickets and you will need reservations and you can't do that until you have your tickets AND these types of changes need to be made in person. By the time you arrive to upgrade the tickets, there may not be park availability for the following days.
You may just have to buy new multi day tickets and then hold onto the ones you have and use them in the future, or apply them to an AP later, etc.
From all reports, the ONLY way to handle ticket upgrades is at the booths in person, unless they change that between now and your trip.