The real A320 use AKH containers, which are special smaller type made specifically for the A320, so that's why we configured all the supported A320s to use that.
If you want to change it, if you change it on the airplane configuration page, you must do it for each major airplane variation (not all liveries), and it will be valid at all airports.
If, instead, you change it on the parking customization page, it will override the setting in the airplane, and will set it for that parking only, or a group of parking spots, or a group of terminals, or the whole airport, but you would still have to do it at each airport.
So, if you really want to stop the A320 from using their own special ULD, everywhere, it's probably easier doing it in the 1st way, and change it for all the A320 you have installed. Again, not all the different liveries, but the major variations (basically, once for each airplane folder)
