I use retrain, and I wish that the a.i. did as well. A few veterans can have a profound influence on a military unit. Those surviving veterans would be promoted to NCO's and officers, and a unit composed of Green troops would respond to their advice, training and leadership. Anyone who has served in the military will attest to the value of veteran NCO's.

Having said that, I usually train at least two units of a particular type that will enter a campaign together. When these units are reduced, they are merged until the campaign is completed. Frankly I prefer three or more units of each type for a lengthy campaign. The result, if the campaign is quick, and the army returns to base with few casualties is that everyone gets retrained with a mild boost in experience. However after an extensive campaign in which the army has suffered 60% losses, the whole force has to be rebuilt with green troops and only a few units from the original force benefit from the "cheat" of retraining.

If you think about it, elite units remain fairly elite even if they must replace their soldiers. One reason for this is that most armies recruit soldiers from within based upon the recommendation of the officers. Promotion is also related to performance and experience. I doubt that it was completely different in ancient armies. I just role play that an experienced officer can recruit transferring veterans who are trained by his veteran NCO's to be a quality unit. I have seen individual commanders do this IRL, so why not in a game?