CentOS 6 to CentOS 7
Today I looked into the possibility of upgrading CentOS 6 to CentOS 7 in a SkySilk VPS. Initial web searches did not look promising. CentOS apparently had a release upgrade tool some time ago, but since withdrew it and now their official recommendation is to do a fresh install.
From further reading, it seems that RedHat only looked into the prospect of in-place release upgrades with CentOS 6. Prior releases didn't have any other option than a fresh install.
As somebody who is used to a relatively reliable release upgrade procedure with Debian and its descendants, this was a bit surprising. Even Fedora has had an established release upgrade tool for a number of years. My first time upgrading Fedora was performed on Fedora 17, and it went smoothly.
So, my conclusion on reviewing the procedure to upgrade CentOS6 to CentOS 7 is: Don't do it. A fresh install is necessary. In the case of a SkySilk VPS running CentOS 6, this means backing up, deleting the VPS and creating a new one with CentOS 7.