As you will recall from Chapter 4, Data Centers, the snitch tells Cassandra what your network topology looks like, and therefore affects data placement in the cluster. If you haven't inserted any data, or if the change doesn't alter your topology, you can change the snitch without consequence. Otherwise, multiple steps are required as well as a full cluster restart, which will result in downtime.
How do you know if your topology has changed? If you're not adding or removing nodes while changing the snitch, your topology has not changed. Presuming no change, the following procedure should be used to change snitches:
Update your topology properties files, which means
cassandra-topology.propertiesorcassandra-rackdc.properties, depending on which snitch you specify. In the case of the PropertyFileSnitch, make sure all nodes have the same file. ForGossipingPropertyFileSnitchorEC2MultiRegionSnitch, each node should have a file indicating its place in the topology.Update the snitch...