Yeah you can take it back anytime within the 14 day period. I got a black one, but changed my mind and switched it out for a white one.
All they did was check to make sure if it worked, they didn't even check the Knox status in download mode. They might check more or less depending on the person you deal with and the managers policy.
Keep in mind you will have to pay the 35$ restocking fee no matter what. You should get to keep the usb cable and the headphones from the old one. (at least that's how it worked for me and another person I know that returned theirs)
Sent from my SM-N900P