I know of three ways that'll get a player out of a vehicle.
1. Use the action command and force the player to eject. This only works in single player or for the host-client in multiplayer. It does not work for other clients in multiplayer.
unit1 action ["eject", vehicle1]
2. Use the setPos command. Simply, set the player's position to anywhere on the map, and immediately move him into the new vehicle. However, setPos causes an undesired side-effect. From my experience, when setPos is used in this way, the player can no longer switch to a missile/rocket launcher weapon. If you don't plan on using any such weapons, then by all means use this. Otherwise, try option #3.
3. Delete the first vehicle with the deleteVehicle command, which will throw out any occupants. The drawback is obviously that the vehicle will be gone from the game. You can always create a new one in the place of the deleted one, but the stats (damage, fuel, and ammo) will not match, unless you set them.