I dont understand ur question...So if like a grunt has level 1 critical strike and you use a upgrade at a building you want the grunts critical strike to go up to level 2?
As far as I know, adding an ability to a unit, be it multi-lvled, is not levelable. The work-around can be removing the ability and adding the same ability that has the powers of the next lvl.
You can only set the upgrade level of a unit via triggers is if that upgrade was based off the unit's default useable upgrades. Else, it can only go to level 1. I hope that made sense?