http://www.ageod-forum.com/showthread.php?t=9481
If I have it right:
1) Volunteers (Militia) apear based on events? and are considered as (Militia) units.
2) Volunteers (Militia) upgrade to Militia (Militia) units based on a % chance per turn.
3) Militia (Militia) units, including those purchased as reinforcements, upgrade to Infantry (Conscripts)(Line Infantry), again based on a % chance per turn.
4) Infantry (Conscripts)(Line Infantry) upgrade to full infantry (Line Infantry), again based on a % chance per turn, OR, can be upgraded through training with a General who has the appropriate ability.
The Militia (Militia) units available for purchase as reinforcements are not Volunteers (Militia) and would therefore upgrade to Infantry (Conscripts)(Line Infantry)?

Technological upgrades for units is another matter, i.e. early infantry to late infantry. I will cross that bridge when I get to it.