Originally Posted by ashill

Interesting. Your post reminds me that DL did do some reshuffling (having various two-letter buckets for upgrades, RP and RO or something, for example) to free up X, V, and E for new, cheap, more-restricted fares, so it wasn't trivial from an IT point of view for DL either. (In DL's case, X had been a complimentary upgrade bucket before it was made into a cheap revenue fare class.)