In CentOS 7 functionality of yum-plugin-security is a part of yum core.

After the upgrade it will be impossible to undo/redo/rollback to pre-upgrade yum transactions. Please run 'yum history new' after the upgrade to start a new history file.

The way yum groups work has changed in CentOS 7. By default yum treats groups as objects now. Please refer to the documentation for more information.