Perhaps, it can be done this way, simply by removing the button without modifying the regular undead creature files. It seems to always provide the regular upgrade. But if you remove it from the creature file, you won't be able to recruit 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 Horde Campaigns without an 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 things won't be affected.
", so yes, it's better not to touch it for now until we come up with a better way )) Added 2 minutes ago
Well! After 2 days of digging into this remaster mod by Rommy, I finally finished my (necessary) edits! And what I want to say is - a HUGE thank you to Rommy himself! He allowed me to write to him in PM and answered my questions, which I had during these 2 days while I was editing. Thanks to his answers, I was able to easily and simply make all the edits I wanted, and now I'm enjoying the wonderful remaster mod in double measure ))) Thank you again, Rommy, you really helped me a lot with your answers!