If you're going to do it the OCL way, here's the best possible way to do it
http://www.cncseries.com/content/?523
You can at least transfer energy and veterency, and you can do it such a way that you can introduce a transition animation.
A more suitable way may be to use the combat bike + Overlord upgrade OCL approach, the Overlord upgrades work by using OCL which OCL the turret inside the tank, you can use this to OCL a dummy rider inside a unit with combat bike behaviour so as to change its weapons, commandset and locomotor while retaining everything else, the dummy rider should have a deletion module set to 0 milliseconds and have some sort of a locomotor (locomotor is necessary for allowing the rider to exit the vehicle properly, if it doesn't have a locomotor you get weird situations where its exited but still inside some how), the deletion doesn't activate until it has exited.
Also, OCLing another object inside a unit that has one inside already forces the one inside to exit.