Hey, just so you know, I had a similar problem with something I was doing.
What I did was create a UNIT called "Please upgrade this" or similar, and it was an invisible unit placed for the player somewhere when they met their upgrade condition, and the unit was removed when they needed to do something again.
Hope you know where I'm coming from, but basically, just use a unit that you can remove and call him whatever you want your upgrade to be called, make him use no pop and be a locust with no attack or anything, that should work for you!
Cheers,
~Gloom