Sure can. I do it all the time. Doing it again this weekend. Use it to get into a park first (so you don't lost the discount) then upgrade it. I have the best luck at the Customer Service in Epcot. Have fun! Cheshire Figment has a great sticky about tickets.
If you go to the locked sticky Everything About WDW Tickets in the TPA&S Forum, especially Post #7, you can get a lot more information. An easy way to get to the sticky is by clicking on the link in my signature.