Did your fare class change to WU?
I got a similar email upon booking a flight for next week (outbound on 5/16). I booked 24 hours before the normal F upgrades go through, but got the "Medallion Upgrade" email and Amex sent me the obligatory "there has been a change to your flights" because of the "upgrade" to C+.
If you are pre-5/16, then I don't know. Is your return after?