coming back to the begining (as BeBro),
I think It would be great to have :
- an Upgrade system
to improve a unit in its type class (INF, CAV, ...) as in HOI2.
- an Evolution system
to change the type class of a unit. I would be limited by a "Service" category ("Cavalry/Armoured", Infantry, Fighter/Interceptor, Bomber).
Germany had unit growing ( GD from inf to mot to Panzer to PZGren) and reduce à the end of the war (they were not willing but ...)
France had Active DI becoming DIM (mot) in 30s.
US had NG becoming "Active" Unit
...
Losing XP is (or not) an impact of these evolution. Soldier don't loos XP during Reforming. They loose Organisation, capacity to "follow the move", knowing what is expected from their sub unit.
-> So a lost of 10 to 20 Xp could be a good thing.
[better system must be found]
[Never mind the manpower complement not veteran that integrat the unit]
- an Formation system
All army had "Depot" unit back home to manage reserves for units.
Using this system, an army with lots of elites units (big Xp) could improve the training (Xp) of a part of their reinforcement.
This system helped Napoleon in this time and the German Army in WWII. This system broke when the armies took too much damage (Spain and Russia for N, East Front for Germany). Recruit going to the front without good training with veteran becoming less efficient and dying more quickly.