I'm thinking that when an upgrade is made, it should give access to new units, technologies if specified, but if affecting statistics, it should only give units access to get the upgrade. for example, you research shields and gain access to the upgrade, then you click on a swordman and you are able to upgrade his shield and raise his armor, however it has not affected any other swordmen till they are upgrades, and you also have to pay for the unit upgrade itself. It would be realistic because it takes varying time for different units to get new equipment