SC2Mapster

Use a Unit Type validator or get the morph applied effects to add a Buff that requirements use.

Maybe just get it to use the cancel for the ability? Use a Set effect that issues it to use all morphs but validate them with a Unit Type validator.

Use Unit Compare Order Queue validators for the Switch effect. That way it will cancel queued units until it has no units queueud then tell it to morph and destroy the persistent.

Use a Create Persistent effect which uses a Switch effect which uses an Issue Order effect to use the Stop ability to clear the queue if there is an ability in the queue and then after a 0.016 sec delay add your morph one once the queue is clear.