A druid has the spell: Survival Instincts which does THIS:
When activated, this ability temporarily grants you 30% of your maximum health for 20 sec while in Bear Form, Cat Form, or Dire Bear Form. After the effect expires, the health is lost.
But when I was in bear form, had it on and switched to aquatic form i still had it. and when i changed back into a big fat tauren, i still had it! This would be ideal in battlegrounds, when changing back to tauren when used.
Screenshot:
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