Some suggestions you may find interesting :
a)At MEL check-in early and politely ask for a seat in Economy Plus; You might have a chance, if not go to b)
b) At MEL ask to the Gate Agent what are your odds of being moved to Economy Plus...
c) When you're already Premier, at the States, ask reservations to erase your Mileage Plus number and insert it again. That's how the system acknowledges you're a Premier. Ask a seat in Economy Plus for your next flights on that reservation.
d) When at the airport, the next time, you'll be all set.

You'll definitely enjoy Economy Plus ! It's magnificent, fantastic and you'll love every inch of legroom you'll get !

PS-I find it hard for you to get advance Economy Plus seats, based on your future earned status, as usually the agent, based on your "no-current-status" won't be able to "see" available seats in the Economy Plus area of the aircraft.