And Delta doesn't need to change anything. They have effectively moved F upgrades into the unicorn category: they still promise unlimited upgrades, but have successfully shifted almost all of FC into revenue seats.

I do wonder, though: if anecdotal reports are true, DL could possibly (IANAL) be wading into dangerous waters, by advertising cheaper upgrade prices based on Medallion status (i.e. offering FC upgrades more cheaply to lower Medallion tiers), while promising DM/PM more upgrade opportunities.