Question about this guy if anyone knows.

So I get that his Atk and Def change depending on if he's in shield or sword form. Is it JUST his base stats that switch, or is it the entire Def and Atk stats that switch.

Meaning, in order to maximize his Atk while in sword form, should you choose an Atk boosting nature and super train Atk (only base stats switch), or should you choose a Def boosting nature and super train Def (Atk and Def switch entirely)?