If I have a Flash Lite unit, can I upgrade that to a full fledged Flash unit by just changing out the cartridge? The restrictive part is just in the cartridge....Right??

i beleive with lite you need a key code

You have to default it completely by draining the cap then you can use another flash card. Use a flash light bulb to short the cap. NEVER short it with just a wire or a pair of pliers.