How do I change this at CWOP?
I know what you mean but your explanation is not quite accurate.
CWOP doesn't need your MAC address.
You are using a 3rd party service called AmbientCWOP.com. This service is not owned nor operated by Ambient nor by CWOP. It is AmbientCWOP.com (a middle man) that you needed to update with your new MAC address.
History (breif) of AmbientCWOP.com:
A very nice guy named Daniel wanted to find a way to upload to CWOP with his Ambient station. Rather than spend money or require additional hardware or local software to gain connectivity capable of uploading to CWOP, he decided instead to build his own software (in the cloud). He designed his software as a cloud service and it pulls and asks Ambientweather.net for data that his station uploaded there. Then his cloud service uploads this data to CWOP. He liked his software so much and he is such a nice guy that he decided to share it for free with everyone (please donate to him). At first Ambient took issue with the use of their name in "AmbientCWOP" and lawyers got involved. It was a tense moment for short bit. But then Daniel was granted the right to use the AmbientCWOP name as long as there was a clear disclosure on the site that the service was not affiliated with Ambient (nor CWOP for that matter).
Please realize that many of my responses on this forum are not always created as direct responses to the OP. My goal is to ensure that anyone reading this public forum not walk away with the wrong idea or incomplete information or vague information.
