We had a similar situation and what we did was first migrate the 1.1 cluster to GossipingPropertyFileSnitch, making sure that for each node we specified the correct availability zone as the rack in the cassandra-rackdc.properties. In this way, the GossipingPropertyFileSnitch is equivalent to the EC2MultiRegionSnitch, so the data location does not change and no repair is needed afterwards. So, if your nodes are located in the us-east-1e AZ, your
cassandra-rackdc.properties should look like:
After this step is complete on all nodes, then you can add a new datacenter specifying different dc and rack on the cassandra-rackdc.properties of the new DC. Make sure you upgrade your initial datacenter to 1.2 before adding a new datacenter with vnodes enabled (of course).