Kafka broker configuration includes a rack label to define the location of the broker.
This is useful when placing replicas across the cluster to ensure replicas are spread across locations as evenly as possible.
This label may need to change for different reasons. One I found today is if you want to redefine locations to support Multi-Region clusters and Observers.
If there are existing topics, and previous rack labels were already used when creating the topics, then the placement may not match the new topology.
Looks like confluent-balancer covers this as one of the criterias to move partition replicas across the cluster, so after re-labeling the brokers with the new broker.rack configurations, applying the change to existing topics is as easy as:
confluent-rebalancer execute \
--bootstrap-server localhost:9092 \
--metrics-bootstrap-server localhost:9092 \
--throttle 10000000 \
--verbose
Docs: https://docs.confluent.io/platform/current/kafka/rebalancer/index.html#execute-the-rebalancer