I had the same problem last year.....
I was a long time seasonal passholder, for about 9 years. I was planning to go to Orlando for a non-disney related function in the middle of July (during the black-out time). But, I decided to go to Disney for a little vacation. So, I wanted to upgrade from a Seasonal to an Annual Pass. I called the Passholder phone number (1-407-560-PASS) and I talked to the representative on the phone. I told them that I wanted to upgrade my Seasonal to a Annual, and I would like the renewal rate of the Annual Pass. He put me on hold, and then came back and said it can be done.
So, I got an upgrade from a Seasonal to an Annual at the Renewal price of an Annual.
Wait for your renewal time, which is usually a month before your expiration or anniversary date, then call the passholder number. Good luck!