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.

In other news... If you can get a unit of peasants up to having gold chevrons, you deserve a medal.

Seriously, they just can't kill anything well enough to pile up enough kills, and die ridiculously fast too. My observations suggest each man in the unit will need ~25 kills to his name to make the first gold chevron level, which for a full peasant unit is 25 x 75 = 1875 kills. That's like... a stack and a half of troops. And you have to keep the peasants that did it alive, b/c it appears that each man's kills are tracked individually. If you lose one that had 20 kills, those kills are gone from the count. Good luck.

Honestly I've never seen a peasant unit that even got to silver...