Currently to turn a Light Infantry Brigade into a unit of Motorized Light Infantry Brigade, Mg Infantry brigade or Siege Infantry Brigade (adding some trucks, Mg's or Artillery respectively) I have to:
1. Disband the unit
2. Go foward a turn so the soldiers are send back to the SHQ
3. Form the new Brigade using SHQ Troops for the Infantry part of the new unit
4. Wait a turn for the unit to become moveable
5. Move the whole unit back to where the original was disbanded
So at least 3 turns just to add some Artillery, MG's or Trucks to a unit. Something that with Turns taking half a season, should really not be taking that long. Something that lets a frontline undefended for at least 3 turns.
That seems excessive, for "adding some Trucks" to drive around infantry.
One option would be allow us to change formations within the same size and base type. Just change it from "Light Infantry Brigade" to "MG Infantry brigade" and have the MG's be delivered by normal reinforcements and some Infantry send back to SHQ (propably the least experienced for each batallion).
Another option would be a rework of the whole OOB system:
You got a handfull of "base unit types" wich have options that you can toggle on/off.
The base type would be "Light Infantry"
One toggleable option would be "Light MG's", wich replaces 1k Infatry with 1k Machine Guns
Another would be "Artillery", wich replaces 1k Infantry with 50 Artillery
And a third would be "Motorized", wich adds enough trucks to move all those people quickly
Most options would be combineable (like the later Grenadier versions, wich have something of everything)
Some options would exclude one another (can not be light and heavy MG at the same time). My best advise would be to have "slots" or "groups" on the options:
There is only one "MG" Group/Slot. Both heavy and light MG options fill it. Only one that fills it can be enabeled. Both Mechanized, Motorized and Bycicles would fill the "mounted Infantry" or "mobile infantry"* slot. Indeed you could even add a "Biological Mounts" option, for horses or analogues.
Of course the whole thing should not be too trivial. A Readiness reduction is the least it should cost, so it is not done without care at any time.
And as readiness also acts as a cap on movement, just adding and removing trucks on a whim is not really that helpfull (nevermind the supply drain of having to add and take back those units and their fuel all the time).
*Mobile Infantry is easily confused with Motorized infantery, a specific form of Mobile Infantry.