Apparently RTW tracks individual soldiers' experience; this is the same system as used in MTW. So retraining effectively only adds new, un-experienced, troops to the unit. The chevron indicating the units' experience is an average of the whole unit.

I.E Retraining a 80 man unit, that has 1 chevron of experience but is down to just 20 men, will add 60 "green" men to the unit; therefore the chevron would disappear to indicate the average experience of the group.

I also believe there is a staggering system (for want of a better description) in gaining experience. It is easier to gain the first bronze chevron than to earn the last gold one. I don't know the details but i believe the number of kills needed to gain the next level of experience is an exponential curve, the more experience you have the harder it is to gain the next level.

Hope this explains how some of the current system works, as stated above i am not 100% on details so if anyone would like to contradict me (or just hurl general abuse at me) feel free.

*Ringo*