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
I'm starting to guess that it might be married segment availability in the fare classes being used for the upgrade, although it can be very expensive to break a through fare when a ticket is upfared.
It would help to know the exact fare codes and fare construction of the OP's current ticket as well as those of the proposed ticket after the upfare for the upgrade.