Properly scaling up your Cassandra cluster is not a difficult process, but it does require you to carefully follow established procedures to avoid undesirable side effects. There are two general approaches to consider:
Upgrade in place: Upgrading in place involves taking each node out of the ring, one at a time, bringing its new replacement online, and allowing the new node to bootstrap. This choice makes the most sense if a subset of your cluster needs upgrading rather than an entire data center. To upgrade an entire data center, it might be preferable to allow replication to automatically build the new nodes. This assumes, of course, that your replication factor is greater than one.
Using data center replication: Since Cassandra already supports bringing up another data center via replication, you can use this mechanism to populate your new hardware with existing data and then switch to the new data center when replication is complete.