A Relay is simply a device which can make its contacts change when the current flowing through its coil exceeds pick up value. A Relay may have NO and NC contact which can be used as per convenience.
As far as Change over Relay is concerned, suppose you use a Relay having 1 NC and 1 NO contact. Also NC contact is used to extend power supply to meter (say) and the relay is energized. Now when the Relay de-energies its NC contact will be NO and No will be NC and therefore the power will now be extended from other source to meter. So the Relay works as Change over Relay.