Everything should link up on
MDE, as long as you've got both resorts and your ticket on MDE (we did split stays in July and again in August and had no troubles). Only potential problem I would see is that if you only have five days on your ticket before you go, then you'll be limited to making five days of FastPass reservations....and won't be able to do the other five days until you upgrade to the 10-day pass. If that's not a concern, then you're probably smart waiting to upgrade until you get there and know you need it. Although those last few days are dirt cheap (relatively speaking), they're still not free! We were down there with some friends who added days as they went along, which turned out good for them, as we were all down there for non-Disney purposes, too, so they would have ended up losing a couple of days had they bought everything in advance...and after the first few days of FP reservations, the last few days didn't seem quite as important.
