It will probably be like what you did with royal knights in MTW when there was a period change,

e.x. with VI, by the year 1206, all the royal knight units were upgraded in the training parchment to their haevier while on the Camp map, I still had a bunch of the old ones, so all I did was retrain these units of Royal knights from the old Early Period, and they came out as royal knights of the High Period.