That error seems to indicate that the gpg key is installed on your box.
It is not matching with the mirror you are using.
The first thing to do is try a different mirror.
If the gpg had changed it should have been upgraded in your system at the time of the change. It is possible that it didn't though. If you have the same problem with other mirrors you should reinstall the gpg key in question.
This is a good source of inforation on dealing with Ubuntu 12.04;