I read in http://wiki.centos.org/TipsAndTricks/CentOSUpgradeTool
"Warning: use of this tool is currently not recommended as several system- critical packages are of a higher version number in CentOS 6.6 than they are in CentOS 7 so those do not get upgraded correctly. This renders yum and several other system tools non-functional."
Does this still hold? It seems to me a bit pointless to offer a tool with the warning that it does not work.