Replacing a Hero, not a Unit

Everybody remember when Arthas found Frostmourne and his model changed, but yet, he still had his old stats and items? I'm just curious. Is there any way to do this?

I've tried Replace Unit but all it does is just replaces the unti with the new unit's model, HP, and Mana. Nothing more.

i think you need to save your "current character" into a variable in a game cache. If you want to know for certain how blizzard did it, get winmpq, open up the warcraft mpq with it, and open the specific campaign file where arthas gets frostmourne. The campaign files are saved as *.w3m btw.