Multiple Effects

Only one spell or effect with the polymorph descriptor can affect a creature at a time. If you’re the willing target of a polymorph effect while you’re polymorphed, the new effect changes your form and ends the older effect. While you are polymorphed, if you fail the saving throw against a polymorph effect for which you are an unwilling target, the new effect changes your form and ends the older effect. The same rules apply to a shapechanger currently in a shape provided by the change shape ability. A shapechanger can end a polymorph effect on itself by using its change shape ability to transform into another form allowed by that ability.