Incidentally I asked a MPC rep recently and was told that it used to be the same bucket but now it has changed. They are from different buckets, I guess like UA etc?

Just to dig up an old thread and add some current info, I just upgraded a set of flights from M (Econ) class to T (Y+) class. Before I called MPC to upgrade, when I checked on the website, there was NO full redemption available in Y+ on either leg.

When I called MPC to process the upgrade, I was able to get confirmed Y+ seats on both legs immediately.

So - it does seem that although fare class is the same letter between full redemption and upgrades, the inventory certainly doesn't come out of the same pool.