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Perhaps, it can be done this way: simply remove the button without modifying the standard undead creature files. It seems to always provide the standard upgrade. But if you remove it from the creature file, you won't be able to hire them in the castle.
Well, it's still a minor issue. Whether you remove it from the creatures or remove the button, how will you play the Lords of the Realm campaign without the alternate upgrade? Perhaps it's still better not to touch it. Or, at the map level, use settings and scripts to prohibit it; then, the unnecessary part won't be affected.
Well, it's still a minor issue. Whether you remove it from the creatures or remove the button, how will you play the Lords of the Realm campaign without the alternate upgrade? Perhaps it's still better not to touch it. Or, at the map level, use settings and scripts to prohibit it; then, the unnecessary part won't be affected.