Modify the upgrade of Roman units?

Is there the possibility to modify the upgrade of Roman units in the following way?

1) delete the ability to build Principes and Triarier in the garrisons
2) add the ability to upgrade Hastati units to Principes units when they reach a certain step of experience
3) add the ability to upgrade Principes units to Triarier units when they reach a certain step of experience

With this modification Triarier would be the most experienced troops in a legion as it was in history and not the most unexperienced as it is right now, as Triarier are in the third battle line and join the fighting late.

and:

Is there the possibility to upgrade Hastati, Principes and Triarier to Roman legionairs after Marius reforms instead to release them all and build fresh, inexperienced troops?

Welcome to the forum Flagg, unfortunately the answer to your first question is no!

You can't change a unit to a different type or description by re-training it - or modify the game to do that.

You can alter the units starting statistics in export_descr_units.txt
and which building they are built from in export_descr_buildings.txt, if you want Triarii to be a better / later unit.
But unfortunately converting units once built is not possible.