EVs will stay the same, but base stats will change.
So if you're running 252 HP / 252 Def and you switch to Atk stance, your EVs will remain the same, but your base stats will change.
For that set I'd say go Bulky Physical attacker with maybe some SDef. 252 HP / 114 SDef / rest in attack. King's Shield gives you your physical Def, while you have some excellent bulk, and protection in SDef. SDance, attack and Bam, massive attack stat all set to sweep.