That's exactly what I just did.  My current AP expires in October.  I'd planned on buying a new AP when I go the end of January-NOT renewing this one.  I was able to purchase a new AP, but when I go in late September, I think I'll make a stop at GS first to make sure that my park entry won't start my new AP.   The CM on the phone told me that I'd been sold a voucher, and that it wouldn't activate until I go to GS to activate it-I just want to make sure.  Oh-and I got my $75 gift certificate an hour after our phone call.