Just wanted to echo the advice of knowing how much you should be charged for the upgrade taking the price-bridging into consideration. On our 2011 trip, DH and I bought our PH tickets through a discounted website associated with my employer. After purchasing the tickets, we figured that we could save some money by upgrading to an AP because we knew we would want to take DS back and as long as we did so within a year, with the discounts (free parking namely, since we stay offsite and would have to pay) it would be worth it. Then we decided to add in a trip to DL within that year, so it ended up working out that a Premier pass would be worth it for us.
On the first day of our 2011 trip, we went into MK for the evening just for dinner and a couple of attractions, then went to upgrade our tickets to the Premier AP (BTW we tried to do this at City Hall in MK and were told it could only be done at the ticket counter outside the park). The CM at first did not bridge our tickets and I told her that was too much for us to pay based on what I had calculated. I mentioned price bridging specifically, and she ran it past a manager quickly, then did the upgrade no problem with the bridging. She was very nice about the whole process and I think the only reason she did it incorrectly the first time was that she was earning her ears, if I'm remembering correctly.
Long story short, know exactly what you should need to pay, and if they give you a price higher, question it! Or take your tickets back and try to upgrade elsewhere.