If Cabrera played 3B the last 2 years as opposed to moving back to 3B for this season, would he have been less valuable this year than you currently judge him to be?
yes, I guess -- don't you think that moving off of one's customary position to a more difficult spot in order to make room for a teammate who will help the team creates value/is valuable?
I would be very surprised if the Tigers did not approach Cabrera about the move before going ahead and signing Fielder.
It's not a given. There are plenty of examples of superstars refusing to move (Larkin being one).