Carol gave you the exact directions on what to do.
However, I would call ahead of time and ask if you can still upgrade the ticket once it is used. I know this sounds crazy, but a few weeks ago we were at WDW. We met up with friends that are FL residents. Their APs have expired and were not sure they wanted to renew in June. They usually do not go to the parks in the summer so they get FL seasonal passes. So, they were thinking of just buying a 1 day ticket for that day. They were explaining their situation to the CM at the GS office in DTD. The CM told them they could buy the 1 day ticket that day and use it; then if they decided to upgrade AFTER the 1 day pass had been used, they would be given full credit of the price of the 1 day ticket towards the purchase of a FL seasonal AP. We were all shocked and could not believe it would work this way. There was a time limit on how long they had to upgrade. It was either 2 weeks or 30 days, I can not remember. They bought the 1 day tickets and they immediately used them.
We then decided to go back to a park together before the blackout dates started for FL seasonal passes. We meet at the park (Epcot); they went to GS and they were able to upgrade to a FL seasonal AP and were given a full credit for the price paid for the used 1 day ticket!
We were all shocked but very happy. I do not think they "got away" with anything, because we were told this info from a CM at GS in DTD and then had no problem doing this exact thing, at a different location (GS in Epcot) 2 days later.
This may not apply to MYW tickets because they are tied with the resort room and the
DDP. But you never know. It would certainly be worth it to me, to make a phone call. Also if you do call, and get a CM that does not seem to know too much--hang up and call back. Unfortunately the turnover of help is happening so fast they can not get people thoroughly trained before they are put in the line of fire. We have friends that are CMs at WDW that tell us this, all the time.
I even exprienced this personally a few weeks ago. I called the Annual Passholder line to inquire about FL seasonal passes. I was told such a thing did NOT exist! I suggested he check with someone else, and he of course, came back and gave me the requested information.
It is a sad day, when the WDW guests know more than the CMs.
Maybe they should seriously consider raising their hourly wages. They pay pitiful amounts. But, in all fairness, they are in line with the market wages for their field and their area competition.