This worked for me:
kubectl label node cb2.4xyz.couchbase.com node-role.kubernetes.io/worker=worker
NAME STATUS ROLES AGE VERSION
cb2.4xyz.couchbase.com Ready custom,worker 35m v1.11.1
cb3.5xyz.couchbase.com Ready worker 29m v1.11.1
I could not delete/update the old label, but I can live with it.