Based on feedback from the field, we need to document that changing replicas using the Topic Operator is not currently supported. This should be documented in the existing "Creating a topic" procedure. The procedure includes this note:

NOTE: Changing topic names using the KafkaTopic.spec.topicName variable and decreasing partition size using the KafkaTopic.spec.partitions variable is not supported by Kafka.

Nothing is mentioned about changing replicas here.