I just found this thread that shows how to enable the "marian reform" event in order to automatically change bodyguard units after the event. This means that we would be able to change from the Early Bodyguard to the Late Bodyguard midcampaign. I thought this info would be useful since DOTS goes from 1080 to 1450.
http://www.twcenter.net/forums/showthread.php?t=524825