It’s on page 20 of the Europe 2e manual, last paragraph:

Any industrial complex, air base, and/or naval base located in the captured territory is now controlled by your side (see “Liberating a Territory,” on the next page). If you capture an industrial complex, you can’t mobilize new units there until your next turn. **_If you capture a major industrial complex, it is immediately downgraded to a minor one.

This is the only mention I’ve found in the the rules, even though they discuss the damage markers in multiple places.  It would have been helpful to reprint that in the unit profiles section._**