The Triarii were the oldest and most experienced veterans, so wouldn't they presumably be better trained to fight with the gladius in the style of the Principes and Hastati? Was their use of the spear an old vestige of the phalanx system?
Additionally, was there considerable retraining that was needed in becoming a Triarius?
As an addition, was there any kind of system by which men were moved from the hastati to the principes then the triarii? We're men just roughly split based on experience when the legion was formed or were there rigid rules?