wow you played his defense, thats great!
make the dark swarm fallow a small location like 1x1 square location (location 1)
nuetral player brings at least 1 dark swarm to location 1
current player brings at least 1 specific unit to location 1
remove unit at location 1
create unit (upgraded unit) at location 1
preserve trigger
now if you dont want the unit that you created to move... just make a location over a terrain that is not walkable and name it location 2, then put this after create unit.
move unit from location 1 to location 2