I assumed you purchased already.

I recently booked CLE-SEA via IAH.G fare (lowest fare bucket, I think). After booking, it gave me the option of upgrading all four segments for $2300 total. Out of curiosity, I logged off and checked the same itinerary in FC -- resulting fare was $1690, more then $600 cheaper than the upgrade fare! This probably doesn't happen too often, but may be worth while checking paid F before booking.