Take Off MAC and subsequently Take Off Trim setting is calculated on the allocated seating configuration. Change this before getting airbourne and these setting just won't work. If pax move in flt, the autopilot will gradually trim with these small changes and keeps the aircraft in trim, starting landing run "in trim". Hence moving before Takeoff, there is no way to be "in trim" other than using the seating configuration allocated by the airkiosk and subsequently the mass and ballance computers.
Mr L.