Hmmm, I can do it through my iPad, but not through the Mac... Used same key on both. Under 'your account' it does tell me (on both) that I have the key for my shop...
Looks like I needed to clear out my history... All sorted now.
It saved your old one though that also has the same name of your new one. The old shop wouldn't work with the new town, thus the issue on your Mac.
