Stance Change switches the attack and defense of the Pokemon with the ability and it happens right before Aegislash uses an attack. It cannot be stopped by moves like Simple Beam and will revert back to the defensive mode upon the use of King's Shield.
So if you're asking when it goes from Shield Form to Blade Form, it changes at the right before you attack of the turn, then yes.
If you had Aegislash and I had Delphox, you would use Shadow Ball and Aegislash would become Blade Form and have its stats reversed and then my Delphox would attack first and defeat your Aegislash because its faster and Aegislash now has frailer defenses.
Also, Aegislash will revert to Shield Form when it uses King's Shield.
Hope I helped even just a little bit!