During a discussion with the CAA today about the perils of navigating round a chunk of CAS when you cannot get a clearance, I compared how GA navigates compared with your average Boeing or Airbus.  In an airliner you might typically load the flight plan in the FMC/FMGC  then copy that into the secondary flight plan and amend and save it to take account of an alternative routing you might get.  You could save that just in case.  Then if ATC decide to change your expected route you could just activate the secondary, select GOTO the next waypoint in the new route and away you go.