I believe what you are doing is called command sourcing, we did the same at the beginning using Akka Kafka and Kafka streams. After 4 years, we're now almost fully migrated to using full event sourcing using just kafka streams.

I also believe you can't have rolling ugrades in Kafka streams but if you have the KIP please post it. https://stackoverflow.com/questions/48667706/kafka-streams-application-updates#53115605