I dunno, but doesn't the CoL need to constantly ensure all armies are up to strength? but I think the CoL needs the commander of the legion to request.
anyway, I think as Asina, I did that once, I moved 3 half-strength units from Legio II over to Arretium to retrain it, but it didn't cause much hindrance, because it rejoined the rest of the army to march to Segesta the following turn