Oh, jeez. You ascertained this all from a "smile". The vast majority of FA's probably don't even know what a 'K' fare is (let alone would they be provided knowledge about future changes in the domestic upgrade program). Sorry, but I'll take your speculation with a large grain of salt.
Amen.
Delta once offered upgrades only on K+ and it worked for them for a long time but it would put them at a major competitive disadvantage today and that is the only reason that it changed from before.