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
When you upgrade (EUA, miles, or SWU), you receive miles based on the fare you actually paid. In this scenario, you would earn the usual miles for a K fare, in particular 100% MQM but not the 150% MQM of a FC fare (unless you buy a M ticket, which is unnecessary here).
It would be wise to be sure that the upgrade is available before purchasing the ticket.