How should I upgrade Strimzi Kafka Operator (Helm 0.39.0 → latest) and Kafka (3.6.0 → 4.x)?

[tanawatcha]

I’m currently running Strimzi Kafka Operator installed via Helm chart 0.39.0, with Kafka version 3.6.0 in production.

I would like to:
• Upgrade Kafka Operator to the latest available version.
• Upgrade Kafka brokers from 3.6.0 to the latest Kafka 4.x.x release.

Could you please provide guidance on the recommended step-by-step upgrade process?
• Do I need to upgrade the operator first, then Kafka?
• Any compatibility concerns or CRD changes I should be aware of?
• Best practices to minimize downtime and ensure a safe rolling upgrade?

[ppatierno]

The CRDs are compatible so there are no issues from this perspective.
In your case, if you want to move to KRaft as well, you should update the operator first to 0.45.1 and the Kafka 4.0.0. Then doing the KRaft migration. After that you can upgrade the operator to the latest 0.48.0 and Kafka 4.1.0 if you like.
Instead, if you want to stay with ZooKeeper you can stay on 0.45.1 which is the last version with ZooKeeper support.
In general, upgrading the operator comes first upgrading Kafka. It raises warning because it can't handle an older Kafka version but after upgrading Kafka these warnings are cleared.