ITT: Change Deoxys to make it be able to change form mid-battle.

Also, would such a thing make it significantly better or worse?

I would probably do it in the form of an ability that changes form based on what move its using.
>always sent out in normal form
>if using most attacking moves, goes into attack form
>if using recover, taunt, self buffs or other defensively inclined moves, goes into defense form
>extreme speed, double team, agility, priority moves, and some other attacks, goes into speed

Frankly, it'd probably be a strong nerf, but it'd be neat to see as a hidden ability or something.