I asked this same question for my trip this past May. Our AP expired three days after we were to check in and we didn't want to renew this year. I called Disney about this and got (surprise, surprise) 2 different answers in 2 different phone calls. The first one was that as long as it was active on the check in date, I would get the rate for the whole stay. Fine by me.
The second answer (from a supervisor) was that I would get the rate on a provisional basis at check in. i.e., the first three nights would be checked in at the AP rate. The remaining two nights would be granted the AP rate provisionally. I would need to renew the AP and then come back to the desk at some point and show the renewal in order to guarantee the rate. If I did not do this, I would be charged rack. This sounded screwy to me, but whatever -- I think this must be "technically" the policy as it sounded like she was reading from a book!
I never did get the two answers to agree, so we left mentally prepared to do battle with the front desk and/or change our ressies upon arrival and spend the last 2 nights off property. In the end it didn't matter bacause no one asked for the AP (I think only once in five years of being AP holders have we ever been asked, but still.... we like to be within the rules) and the rate was never questioned.
To make a long story short, I never did get an answer to the question, but it worked out for us.
JW