Yes, it's handled internally, but you can cheat. You can set a unit's experience, level, and adjust its promotions, which makes it fairly easy to copy an existing unit's promotions and such over to a newly spawned unit. The Convert function he mentioned obviously does that in a more direct way, but this other method has advantages if you want to make a bunch changes in the copy process. Replacing a ranged promotion with its corresponding melee one is the obvious use, but you might also want something more complex, like having a Vampire unit whose spawn have all of his abilities... except the ability to create more spawn. If it's a single simple change then it's easier to deal with that after-the-fact, but if you've got a large list of things to change that can get prohibitive.