Quote Originally Posted by Foz
The upgrade idea is largely unnecessary. The point of experience is that it makes your troops that have been around a while as good as their later higher-tech counterparts. You don't need them to be the next higher level of troops, because they already get attack, defense, and morale bonuses that make them at least as good in most cases.
While I concur with the later portion of your post, gotta disagree with this ^^

If you had a unit of experienced 'crack' soldiers... would you leave them in their outdated weaponry and armor, because they're at least as good - and maybe a little bit better - than your freshly trained and equipped troops? Of course not. You'd want your best soldiers to have your best weapons and armor also, so that they could truly provide a solid core to base your army around... and they'd want it even more than you.

This is accomplished ingame somewhat with the retraining from Armourers... but last I heard we still weren't certain if that was functioning as described or not.