Rolling Java update in Cassandra

We are currently running Apache Cassandra 1.2.13 on 4 machines with Java 6 and plan to upgrade Java to version 7. Is it possible to upgrade Java on the current cluster, i.e. node to node? I know Thrift protocols for client-server communication are incompatible for versions 6 and 7. for Java. How does this relate to internal server-to-server communication in a cluster?

I haven't mentioned this in the documentation or other sources.