Your reservation has to be made while logged into your Emerald Club account. When you make a reservation it will say "level of service" and then counter, Emerald Aisle, or Emerald Reserve. Simply showing up in the garage with Emerald Club card in hand won't do it......
There has been mixed results in calling and getting your Emerald Club account added to an existing reservation. So, I would join emerald club, then log in and try to make a reservation. If you get the same or better rate, make that reservation and then cancel the old one. If you can't get similar rates, call National and see what they can do for you.
That said, I check my ressies regularly and cancel and rebook anytime the rates go down.....
Duds