If I buy just one battlefield upgrade kit and equip it, would un-equipping it turn it back again?

Think of it more like plugging three different devices into a USB slot. If you remove one, and it is then added to your overall Hangar inventory and you can plug in another. It also seems important here to note that the upgrade kits are permanent items which will appear in your Hangar and not 'single use' tokens. (Another analogy: think of them as like having a set of snow tires for your car. You remove your existing tires and put on the snow tires and can then switch back and forth as the situation warrants.)