Well, with the AI insanely attacking you from everywhere the problem is that there is not really time to ship the veterans home and back.

I remember one game I managed it on M/M, but the current game is VH/H and I too find myself with about a third of each and most of my Camillan Troops (it is 211 BC and I run pure Camillan Army still, no time to ship them home and exchange them) have silver and gold chevrons, but are all 10- 50% strength.