I'm working on a hero whose ultimate spell permanently transforms him, improving all his abilities. I tried giving the alternate unit the improved spells, but while that changed most of the abilities, I still could only learn and level up the base unit's abilities. I also tried adding an ability to the alternate unit based on the Engineering Upgrade, as well as removing the abilities via triggers and adding the new ones after the transformation was complete, but I got similar results.
It's worth noting that the only ability I can't change with any method is a modified Polymorph, and the upgraded version is based on Channel with some triggers to make it aoe.
It's worth noting that the only ability I can't change with any method is a modified Polymorph, and the upgraded version is based on Channel with some triggers to make it aoe.