I am bit confused.
Why are we talking about the award seat availability?
CO ticketed the OP and it's doesn't matter NOW what bucket the fare is. Once you're ticketed only the cabin is all that matters.
Why CO is looking again for the award seats? Shouldn't they be looking for any seats in the cabin that was originally booked? Or they are treating the OP now as the change(on the accepted change) is initiated by the OP?