Originally Posted by pitz

Those 'upgrades' are subject to I class inventory. If you get one, its confirmed. But there isn't often a lot of I class inventory dispatched on desirable flights.

Confirmed eUpgrades come out of R inventory. Unconfirmed/waitlisted eUpgrades are processed out of positive space at flight time per a minorly complicated priority scheme that provides preference for cabin, status, and fare.