Perhaps it's a tech that allows the ability to convert militia to infantry?
That tech is kinda intruiging...
Also, in HoI2, if you had 2 air 'corps' with 4 units each, you couldn't switch units between them since the game only allowed 4 units in one corps for air. This was a buit of a nuisance sometimes when you needed to switch damaged units for fresh ones

I hope the switching of brigades (if it's in) will be easier.
I think the viability of converting militia to infantry is dependent upon one's definition of "militia."
If "militia" equals WW2 German VolksArmee (the old, the very young, the partially disabled; all men not normally eligible for conscription, i.e. the last ditch dregs for local defense) - no, absolutely not.
If "militia" equals WW2 US National Guard and UK Territorials - then OK, sure, convert them to infantry - they were historically, anyway.
HOI3 really needs more forms of ground combatant, IMO. I would like to see the following categories:
1. regular infantry,
2. reserve infantry ("militia" is how I see them labeled now),
3. static infantry (for fortress defenses),
4. partisans (in conquered territories),
5. guerrillas (for internal civil wars and colonial wars), and
6. para-military combatants ("VolksArmee" types of last-ditch, government-organized and raised local defenders).
As to the switching of brigades, I don't personally think it will be anything like forming of air corps in HOI2 - which was essentially the same process as land corps. The construction of divisions is where brigades are manipulated in HOI3, according to all that we've been shown so far. I think that further manipulation/switching of brigades will have to happen in some similar, construction type screen that is far from the battlefield where corps can currently be constructed and re-combined.