I know that for certain units the skill they have changes upon cc them and in very rare cases upon awakening.
What I have noticed though is that for units gold and up that their for awakening is different when they reach max level.

How exactly does this work? Does it simply change upon leveling, does it have to be triggered, does the level of the previous skill
carry over to it?

I am a long ways off before the point comes for me to worry about it but I would like to learn about it heading in.