I just tried out the upgrade tool to upgrade a server from C6.6 to C7.0. Unfortunately the server doesn't some up any more. Screenshot:
http://www.cora.nwra.com/~orion/Screenshot_mailmirror_2014-12-11_16:37:55.pn...
Doesn't get past Welcome to CentOS Linux 7 other than printing something about a dependency cycle.
Will have to look around more tomorrow.