I guess this suggestion might be undo-able in the current game system, but I'd much rather somehow pay more to have the upgrades done automatically at the beginning of the turn they become available. Boom, everybody gets the upgrade, done. No need to lose individual units for three turns over the course of umpteen turns.
Plus and besides and furthermore, historically entire corps weren't removed from the line in order to get an upgrade. If training were required for new equipment, small units or groups of officers would be sent to the training ground and they would impart what the learned on the rest of their units. The whole process can be considered part of the rotation of personnel and equipment in and out of the line that isn't represented in the game [and doesn't need to be].
