http://www.datastax.com/docs/1.0/operations/cluster_management#changing-the-replication-factor

It looks like a nodetool repair is needed after increasing your RF. I guess that a cleanup has to be performed after decreasing the RF to remove useless replica.

Alain

2011/12/5 A J <s5alye@gmail.com>
If I update a keyspace to increase the replication factor; what
happens to existing data for that keyspace ? Does the existing data
get automatically increase its replication ? Or only on a RR or node
repair does the existing data increase its replication factor ?

Thanks.