Documentation is available at https://kpmeen.gitlab.io/kafka-websocket-proxy/.
Prior to version 1.x.x the internal session model only supported consumers. This model is not compatible with the session model used in version 1.x.x.
If Kafka WebSocket Proxy has only ever been used for producer clients, then the upgrade should be unproblematic.
When Kafka WebSocket Proxy has been used for consuming data it is best to ensure that the session topic is empty / recreated on the Kafka cluster before deploying version 1.x.x.