The CORE mod had an interesting tech path that allowed continues motorisation of regular INF throughout the war. Unfortunate downside: Once researched, your INF automatically became equipped with trucks and you never ever could use "pure INF" again.
In general you could just build new units and disband old ones, but being able to upgrade units on the field is... well... much easier and user friendly.
However, what could be done:
Maybe you could add a tech called "Infantry Motorisation" - with no effects. But: Once you have researched this tech, every few weeks an event triggers, that replaces a random INF division with a MOT division (in exchange for supplies and some negative IC modifier).
In general you could just build new units and disband old ones, but being able to upgrade units on the field is... well... much easier and user friendly.
However, what could be done:
Maybe you could add a tech called "Infantry Motorisation" - with no effects. But: Once you have researched this tech, every few weeks an event triggers, that replaces a random INF division with a MOT division (in exchange for supplies and some negative IC modifier).