How to discount an summon from the spell's max unit allowed after the summon morphs

[Eagle XI]

For the nerubians i am using a modified scarab spell, which summons a cocoon out of a nearby corpse, which is also generated by the building. The Cocoon can later morph into a plethora of different units.
The thing is i have to limit their production, and the whole cocoon thing started as a means of that. So the production structure would have this ability on auto-cast and maintain only 3 Cocoons at a time. (blatantly inspired by zerg hatchery mechanic in SC)

Now i must find a way to discount the cocoon from the spell's max units at a time after it has morphed into any of those units, so the auto-cast can continue, for otherwise still 'detects' those morphed units as still living summons and ability stops auto-casting till any unit that have evolved from a cocoon ever summoned by this spell belonging to that production building, have died for good. However you can still cast manually(generate new cocoons) and ability functions as intended otherwise.

 

[Nichilus]

make the cocoon not morph into anything and instead of when it should morph, kill the cocoon and create new unit in its place.