A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
Yes you can decline an upgrade. Just tell the GA. Better yet, tell the GA in advance that you don't want the upgrade unless GF gets upgraded too.
Hard to give odds without knowing details of route, day, and time.
One argument is that if Silver gets UG, FC must be wide open so GF will probably get UG too. OTOH, if Silver gets UG, one can argue that it's likely to be the last FC seat. So, it depends.....