Just a little tip for those that don't know. If you have different levels of experienced troops, 2 chevrons, 3 chevrons or 4 chevrons etc in the same army, retraining them all together will lessen their experience. I have my infantry lines at 240 men per unit. So If I have 2 units of 3 chevron infantry and one has 190 men and the other has 70, I move one stack onto the other stack to replenish their numbers.
Moving the 70 onto the 190 stack will fill up the 190 to 240 leaving 20 in the remaining 3 chevron stack, thus I only have to retrain one 3 chevron stack.
It's advised to do this outside a city or town or it may cause a ctd. Do the same with the 2/4 chevron units but retrain any remaining troops with different chevron levels in different buildings to keep their experience. However, retraining them still takes 2 turns so maybe someone knows what file to edit to retrain them in 1 turn?