I have tried that, but it fails:
400 - Bad Request : REQUEST JSON does not validate against schema
I downloaded the current configuration file from the device (US), then modified the frequencies for each of the channels according to the TTN conf.json but when I upload I get the above error. I have also tried to adjust the frequencies manually using the slider in the web interface.
Without more info, I am guessing the firmware checks the frequencies against the location and rejects them if they differ.