I'm not sure if it's applicable to GUC, or just complimentary upgrades, but you may want to take look at these two threads that talk about loosing upgrades and medallion benefits after you change your FF# on your reservation to Alaska.

Gold Medallion status, booked a delta flight, but don't want to credit to Skymiles...l

http://deltapoints.boardingarea.com/...woman-scorned/