Ok to do that heres how I think: crate 2 units one that looks exatcly like each other and give him metamorph give him locust to then do this
Unit Starts effect of ability:
Ability being cast = cluster rockets
Wait 1.50 Seconds
Replace triggering unit with whatever
order last replaced unit to cast metamorph
remove ability metamorph from last replaced unit
remove ability locust from last replaced unit
Then make sure both units are exactly the same so stats models damage and stuff come out the same I hope this helps