When you did the switch did you just swap out the software on each node, or did you provision new nodes with Scylla, adding them to the cluster, and removing old nodes with Cassandra?
I don't believe you can have a mixed Scylla/Cassandra cluster. In our case we cloned our drives and started a whole new cluster with Scylla. When we were happy with Scylla and done the appropriate testing, we shutdown the Cassandra cluster.