Save your mission once you are sure you will never have to modify again the unit, then quit ofp and go in your hard drive in your ofp folder, go in "user" part and find your mission, then open the missions.sqm file with wordpad or any document prog, then find in the line of the right unit and erase his side part, write another side for replace it...
Save and launch ofp, open the editor, load the mission and save.
(be care: as said zwobot if you edit the unit again in the editor you will disable the effect and make the unit as originaly!)