The camilian & polybian MICs are in one building tree. The marian & imperial are in another. C-->P, nothing happens, you have to upgrade your barracks to get new units. P-->M, the script damages all buildings from the C&P MIC building tree and places a M barracks the same level as the P barracks you had in that town. M-->I, nothing happens, you have to upgrade your barracks to get new units.