At first, I remember I used to always look under the C's for the action move location, since the sentence starts with "Center location..." but the action is actually "Move Location." This would be why you couldn't find it, even though zeratul worded the trigger out for you.
So ya, use the "Move Location" action. It checks a location (very often "anywhere") for the presence of a unit owned by a certain player, and if there is atleast 1 copy of that unit in the location owned by the correct player, it centers the location on the left-most one there.
So in the end, it would possibly looking something like "Center location (2x2) on ssj1 owned by current player at Anywhere."
Then just do, "remove 1 ssj1 at (2x2)," "Create 1 ssj2 at (2x2)."
And zeratul, you can put triggers into the map, if you use put as the action of creating them
I don't think he was ever confused on the idea of triggers and locations, anyways, though.