If you are using createVehicle you can set the side of a soldier.
But once he is created that's it, he can't change side. Best you can do is use setCaptive.
See the command reference in the ed depot if you need more details.
Edit: I suppose you could use deleteVehicle and createVehicle on the same spot to make one loon disappear and another one appear with similar attributes but on the other side.