If you USE Volcano Bay on your 3 park ticket, you can ONLY upgrade to a 3 park AP.
If you buy 3 park but don't go to VB, you can upgrade to whichever you prefer. I asked multiple people while in guest services, and they even called someone else to confirm.
It makes sense, because otherwise everyone would go to VB then downgrade to the 2 park AP and get refunds.
I ended up adding one day to my tix as that's all we needed, and they were able to extend the expiration for me (so, we had 3 park 2+2 day promo tickets, went our 4 days over a 5 day period, then added the extra day at the 3 park 5 day price (~$40/ticket to upgrade), but got that extra day extended to be able to use it 10 days from the first scan as that was the day we needed).