T used to not exist. And only L fares weren't upgradable. Actually, I think before that even L was upgradable. L class was largely a fare class they used to compete with LCCs. They have used that excuse forever and added at least three more fares classes just like it for the supposed same reason. During the period I am walking about this was a PM exclusive benefit and DM didn't exist. This was for domestic. It didn't require any upgrade instrument at all. Even GMs basically got an unlimited number of 800-mile (and later 500-mile) upgrade points. This was in addition to Systemwide Upgrade certificate (SWU) and later North American upgrades which I forget the exact term but I think was referred to as a NAMU.