If we wanted to get really fancy there could be a mode that you enable in the designer where all the gear is unlocked letting you create units for any tech level. After you finish that, you would then get a prompt where you arrange the units you just made into an upgrade tree, with possibilities for branching paths (so you have the ability to turn some into mounted units for instance). Then the AI could easily take advantage of what the player has just done for convenience by using the same upgrade paths.
But that would probably be a lot of work.
