I'd prefer to see retraining being the mechanism for maniple advancement, as this would reflect the age advancement from young limber Velites to wiry veteran Triarii.

Thus:
Retrained Velites become Hastati.
Retrained Hastati become Principes.
Retrained Principes become Triarii.

Failing that, swapping the Principes build option to the Army Barracks in return for moving the Triarii option at the Legion Barracks would better suit the historical shift Rome made from greek-like spear armies to legionari swordsmen. That said I can't remember exactly when the Roman 2nd Line swapped their thrusting spears for pilum, by the time the RTW campaign starts they may already be pilum equipped.