It can't be done in the way that you describe. BUT units can have a property where they change to a different unit when they reach a certain number of HP. It's supposed to be used for things like hydras, that become less dangerous as you cut bits off them. But you can do it in reverse. Set the 'trigger' HP amount to 1 MORE than the unit normally has. When it gains an extra HP due to experience, it will change into a different type of unit - which could then have the same type of transition to yet another type when it gains another HP.
The problem with this method is that in order to apply it to units already in the game, you'd have to change literally every single unit's data. The Populum mod I believe uses this for some units, but not all of them. So it's technically possible, but it's a MASSIVE amount of work if you want it to apply to every unit.

That's clever, thanks for the answer, looking up Populm