The client can modify a consumer using the management functions. The management just passes the request to the server in that case. If an illegal update is attempted, the server will return an error and the client will propagate that to the user.

When making a subscription request, the user is not allowed to modify the consumer at all. Attempting to do so will throw an exception

SUB | 90016 | Existing consumer cannot be modified.

As reference you can see the unit test https://github.com/nats-io/nats.java/blob/main/src/test/java/io/nats/client/impl/JetStreamManagementTests.java

testValidConsumerUpdates shows that you can update these fields

• Deliver Subject
• Ack Wait
• Rate Limit
• Max Ack Pending
• Max Deliver

testInvalidConsumerUpdates shows that you cannot update

• Deliver Policy
• Filter Subject
• Idle Heartbeat