Starting in 1862, units which are upgradable begin...well...upgrading

.
Infantry slowing upgrades to "Late War infantry" which has higher attack and defence fire ratings than early war infantry.
Also cannon begin to upgrade...(something which I don't really like, but is there anyway.)
Cavalry (especially important for the Union) upgrade with rather more deadly off. and def. fire values to simulate the introduction of repeating carbines.
That's about as much as I know. Hope that helps...