I just upgraded my AAA 4-day hoppers to 5-day. It was no problem; they just cancelled out the old order and made a new one. They also gave me the old prices, which are in effect until the end of the year, according to my AAA agent.
I believe if WDTC has 8-day passes, it would be easy to upgrade to those as long as you have enough time before your trip to process it. Just call your AAA agent and ask.
BTW, you can also add on character meal vouchers if you like. The price includes tax and tip and it's nice to have it paid for before you go. It's hard to say how much it will save us, though, as the prices seem to fluctuate quite a bit on the character meals, depending upon when you go! We paid $25 adult and $17 child for the princess dinner for October. The price for any of the character breakfasts is $25/$14. At that time of year it could be better to use a AAA card when you get there (10% off) but it all depends on the going rate for the meal at that time.