First USA has certainly not handled the dissemination of this news very well. The CSRs at Rapid Rewards still say that it's effective for everybody on January 1. My November bill has no info on any change. So the April date makes sense. Sixty days' notice of such a change seems like the bare minimum to me.
I am surprised that Southwest agreed to the $1200 rate when they signed the multi-year renewal with First USA earlier this year. Maybe no other bank would go with the $1000 rate.