There's no guarantee that you can be upgraded after the promotion expires. That being said, I actually DID upgrade from Pop to Caribbean Beach Resort last year even after I had paid off our trip. I had to make a couple of phone calls before it worked though. The first two people didn't act like they really wanted to check the system because all they would say is "that promotion has ended".
The last time I called, my end of the conversation went something like this...."Hi I have an existing reservation using the *** (insert whatever promotion you booked with) promotion and it's already paid in full. However, we have realized that we want to upgrade to the Caribbean Beach Resort. I know that you guys won't have a problem with me sending more money your way. Is there any way that you could check with a supervisor and see if we could upgrade to a moderate resort?"
The castmember was very nice and said let me check. She said she didn't see anything available but could I hold a minute. She came back and asked me if I was ready and willing to upgrade right then and there and to pay the difference right then and there. I told her "absolutely". She put me on hold again. I held for a few minutes and when she came back, she said her supervisor had given her the go-ahead to upgrade me from Pop to Caribbean Beach Resort. I paid the difference with my debit card and everybody was happy!
That being said, I don't know if I just lucked up. It never hurts to try. Keep us posted on how things go for you.