Make a proxy ability of type "effect - instant." This is the ability the player will click that will actually be on the command card.
Make an effect of type "Issue Order", set the unit to Caster and set the ability to your morph ability. Under command flags check "Preempt" and set the proxy ability's effect to this.
What happens is that when the ability is used, it places the morph ability at the top of the unit's order queue but retains the rest of the order queue, so after morphing the unit will resume its previous orders.
Here's what you do:
What happens is that when the ability is used, it places the morph ability at the top of the unit's order queue but retains the rest of the order queue, so after morphing the unit will resume its previous orders.