Not sure about formation flying but as far as I know, you can't change your call sign unless you are in an uncontrolled airport. For example, if I was talking to ATC or landing at an airport with a tower, I would say after initial contact "Cessna 86Mike". However, in uncontrolled airspace or at an airport with no tower, I could say "White Cessna on left downwind runway 24".. I would personally not advocate this but some people do it...