Hero keys, answer if you can plz

So if hero keys are able to be used multiple times, can you constantly switch between hero’s you don’t own?

i think the key unlocks them for good

As far as I understand it, no. Once you use a hero key on somebody, they are unlocked and you cannot get it back - until you complete whatever in-game requirement would unlock them, at which point, they stay unlocked and you are refunded the key to use on somebody else. So the only way to ‘switch’ who you have unlocked is to unlock the hero through normal methods.