enabling, disabling, barracks

It's handy to have a barracks lvl 1 to increase production speed by 25% but the lvl 2 doesn't increase it any more. So when I'm producing something not requiring a lvl 2 it would be handy to disable by steps rather then all lvls at once. To enable for just the speed increase, when having a lvl 2 or 3, costs allot of food.