MSK Upgrade - any concerns?

There are no known compatibility issues from the client perspective. But there is a functional change that may impact current behavior of producers, if/when you decide to update the client's libs as well. For messages with no key presented, the default partitioner in Kafka 3.3 has been changed, and it is not round-robin. Again, this is not a change on Kafka brokers, it's a change in client libs.

Also, I would recommend to run upgrade Kafka version API during off-peak window, making sure you don't have another maintenance scheduled, and having about 2 hours for the operation to complete.