Until someone answers with more details, few questions:
1. did you moved the system keyspace as well ?
2. the gossip IP of the new nodes are the same as the old ones ?
3. which cassandra version are you running ?
If 1. is yes and 2. is no, for a quick fix: take down the cluster, remove system keyspace, bring the cluster up and bootstrap the nodes.
I recently migrated 400 GB of data that was on a different cassandra
cluster (3 node with RF= 3) to a new cluster. I have a 3 node cluster
with replication factor set to three. When I run nodetool ring, it
does not show me all the nodes in the cluster. It always keeps showing
only one node and mentions that it is handling 100% of the load. But
when I look at the logs, the nodes are able to talk to each other via
the gossip protocol. Why does this happen ? Can you tell me what I am
doing wrong ?