Can you be more verbose? What does the current rule break?
Oh I checked, really, no upgrades conform to Sid's Civ rules! Then we should remove unenabled upgade from user interface and AI consideration since the illegal action message just comes out of the blue.
Client part -> #43424
This ticket is left for the AI changes.
(original header was: civ1: UpgradeUnit not enabled)
Action must be enabled in the ruleset. AI can't even pretend it's disabled, btw.