Here's the funny thing...
I just got back from a split stay last weekend. We did 2 nights at OKW first and then 3 nights at BLT. The BLT nights were on the MB test. So, we were not able to book FP+ for the days when we were at OKW (which were right before our BLT checkin.) However, imagine our shock and surprise to find out the day we were leaving OKW that... our MBs WORKED on the OKW door! So I'm wondering if each guest gets their own unique "access code" tied to the door where they are staying? We thought that was VERY interesting!
Hmmm.....
As for future trips, we were told that the MB are ours to keep and we can use them from here on out. We can book FP+ for 60 days out online. We were told by another CM that whoever is getting the MBs now can/should keep them and that they will last for 2 years and after that guests can purchase them for $15.