CO has established a formal process for downgrade compensation. The details can be found in SHARES under GG OVS DOWNGRADE. The gist of it is that you are entitled to a specific TCV comp based on the stage length as well as a refund of miles if used for the upgrade or of cash in the amount of the fare difference between the F ticket you bought and the Y seat that was available at that time. Unfortunately CO management asked that the specific details of this GG be removed from FlyerTalk so we no longer have them explicitly posted here.

Additional information on how CO does downgrades may be found in the following threads: