I bet "officially" they will not be allowed to give you a different model than the one you reserved. BUT, it probably all comes down to the person you are talking to and whether you can kind of charm your way into getting them to give you the 32GB version instead. It also might be contingent on whether they have a healthy stock of ipads, or are on a very razor thin margin compared with the number of reservations they have.
I agree with the other posts that said you should try and ask if they can do it, heck, it can never hurt to try!