Hello there @AvAsac!
I'm sorry to hear that you've encountered trouble with Lt Chang's decryption key. Thanks for taking the time to share your report with us!
You should be given a warning in-game if any settings you decide to change will affect your in-game progress (such as by changing the global difficulty).
Would you be able to provide me with some more details about the issue you've encountered with the decryption key? I'm aware that we have a couple of ongoing investigations into issues with decryption keys. If you can be more spec...