With the new "upgrades to" field you don't even need to use the old custom units method. Specify the unit for it to upgrade to to be another, exactly identical unit. Then setup triggers to detect that upgraded version entering the playable map area, replacing it with one of the old type, and applying the upgrade. This should work better than the unit method because it doesn't create a queue. You may need to use animation triggers to make sure it doesn't play the building animation while upgrading. | 
