When you talk to the gate agent, do not use the word "upgrade". They will see you on an award ticket and end the thought process there. You need to specifically tell them you have "paid" for a first class seat and are seated in economy.

I would check on line daily to see if F saver award space is available, then call and get the ticket adjusted. Perhaps you have already done this and are now asking about day of flight. Be prepared to fly in economy, there are many threads asking about voluntary downgrades being accomodated in the class they paid for, but not many success stories.

Good luck,