I agree you should be able to retrain them to their equivalent level of post marius unit, but personally i disbanded them. The experience my best units had gained by 220-30 something bc (best had about 1 silver chevron) was totally outweighed by the maius units far batter stats. So all of them were disbanded or sent to guard a backwater.

That included all my triarii too, i had only just built my first 2-3 units and they were unused in battle.