I am looking at the code, and I can see what it does, but it doesnt seem to work fully. I can convert t1 medium tanks to variants (the kind you create with army xp), but I cannot convert lower tier equipment to a higher tier. I started a new game and am trying to convert interwar fighters to 1936 fighters, but the conversion button does not even show up in the production screen.

I cannot convert foreign equipment at all either. I dont have any mods that expand or add new equipment. I dont understand how your screenshts show that you can convert equipment to a higher tier...