Since barracks produce veteran units, upgrading conscript and regular units (with the use of barracks) should produce veteran units as well.

Perhaps it should cost more to upgrade a conscript unit than a regular -- and regular units more than veteran.

I think its pretty much fine the way it is, I wouldn't advocate any changes. Besides, then you'd have the AI upgrading all of its conscripts when you're seiging its city, and then you'd have quite a nightmarish situation..

well, veteran/eliete is supossed to represent combat experience, and knowledge gained on the battle field/acadamy. plus the units produced have more than 1 turn to learn the tactics (while it's bieng built vs. the 1 turn to upgrade)