Originally Posted by
Hartmann
Yes, it is capacity controlled. R needs to be available. If R becomes available between now and your trip, you should be able to upgrade.
It is ZE, not R, inventory that the M-Ups come out of. It tracks VERY close to Z inventory. If you see Z inventory then the M-Up should be available. ExpertFlyer can track and alert on Z inventory or you can just check every now and then.