Oh, sorry, I misunderstood your problem. Well, yeah, the ability doesn't take stats from the target unit-type, the original unit is still under the hood. What you can do is give the units different abilities. You could for example use item ability Hero Stat Boost (all stats), change it to give 100 stats and add it to the demon form unit.
The change of primary attribute is a bit trickier and you would probably have to trigger it, because I don't think there even exists a function to change it, your only chance is to have two different units and replace them via a trigger.