I have found: there is effectively a technical problem in the mod: the upgrade path from cold chariot to cold knight is ok, BUT the cold chariot is not marked as upgradable so the path is not active
To change it, use the civeditor:
- open your WH2.5.biq
- in the "rules" menu select "edit rules"
- go in the "units" section
- in the main menu search "cold one chariot" and when you're on, chek "upgrade unit" in "special actions" on the right
The good point is that it will certainly correct the problem. But the bad point is that it will occur only in futur games, not in your current one! Because the changes you make in the "rules" have no effect on saved games, even if they com from this .biq