If you create a new cluster and mimic the tokens across less nodes you will still have downtime/missing data between the point when you copy all the SSTables across and any new writes to the old cluster after you take the copy. Only way to really do this effectively is to do a DC migration. Brief run through would be to:
1. Bring up a new DC (ensure you are using NetworkTopologyStrategy beforehand) 2. Change your clients to only query the old DC (load balancing policy + local consistency levels) 3. Replicate the keyspace across to the new DC 4. Rebuild the nodes in the new DC from the old DC 5. Run a repair 6. Point clients at new DC 7. Remove replication from old DC 8. Split the DC's into two clusters (firewall rules are helpful here).
