Disabling upgrades/alternative upgrades.

Today, I saw a mod that attempted to recreate creatures from Heroes of Might and Magic III in Heroes of Might and Magic V. The gimmick was that alt-upgrades were completely absent; instead of them, there were empty boxes in the hiring menu, and upon upgrading, they immediately received the upgrade without a choice. How can I create something similar, and is it possible to do it partially (i.e., remove the alt-upgrade from one creature from the same faction, remove the upgrade from another creature, and leave everything as is for the third)?