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: 100,417
I suspect it won't work because the flight is full and DL is no longer willing to sell tickets, but have you tried to do a dummy booking when checked in to see whether upgrade and "upgrade" (to FC and C+ respectively) inventory is currently available?
Obviously there cannot be 24 C+ seats available if only 11 show on the seat map, but DL could be in the middle of an aircraft change.