Sorry if a mod like this already exists, I've searched the vault and didn't find it. It seems like it should be a fairly simple thing to write (?) but it's been over a decade since I did any FA modding myself and I don't remember a thing about how to do it.
You can disable the upgrade in the unit manager by clicking the upgrade icon. It'll unrank your game but a mod would do that anyway.
I didn't know that -- thanks!