There is an upgrade tool that allows for in-place upgrades from CentOS Linux 6 to CentOS Linux 7. This tool is Community Maintained, and information is available on the CentOS Wiki and on the CentOS Mailing List.
We currently do not have anyone from the Community maintaining the package, and in its current state it no longer works.
We really need someone from the CentOS Community to step up and maintain this Upgrade Tool, or we are going to have to remove it from the downloads area, since in it current state it can break people's machines if they try to use it.
If anyone would like to maintain the Upgrade Tool, please reply to the thread on the general CentOS Mailing List. (or you can contact me directly at johnny AT centos DOT org