Yes you keep already built units but you cannot retrain them ever again and cannot build more Camillian units again. You will need to upgrade your barracks in order to use the new units. If all your barracks were already at a high level this can take up to 15 turns until you can build your new factional units. Keep this in mind if you are in a heavy war and might need units soon.

As far as I know only the factional MIC is changed therefore regionals should be identical to before.