You mean Light to Medium Upgrade?
Just research the techs and they will upgrade if you allocate enough IC.
No, this isn't working in my installation either. I have the techs (medium tank Pz III) and 12 L-ARM to upgrade, Land Divisions list is showing no applicable upgrades.
Furthermore the switch which controls this is missing in the 1.3b9 version of the setup program, but is there in the 1.0 version.
If I'm going to be editing ironcross.cfg, which one is it?