It's been confirmed that if you own ArmA 2 OA on steam you can no longer change your key as Steam will change it back anyways and will only launch using the key that you purchased. There isn't a way as of yet to solve this except for making a new steam account and rebuying the game, I personally have not bought the game on Steam and I can change my key fine without it resetting. Currently no way to solve this issue, and I don't think there will ever be.