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Okay so.
Mega evolutions and alternate formes can't have HP base stat changes because it would mean their total hp changes mid battle and that conflicts with the way that base stats are programmed.
But, why do pokemon that have strictly out of battle forme changes also have to abide by this rule? Deoxys for instance can't change formes mid battle only outside it. If evolutions (which don't take place in battles specifically to avoid this problem) are comfortable changing the hp stat then why not forme changes too?
