Can I make changes to a queue after it has been created?

Unfortunately not, queues are immutable.

Your only option therefore is as follows:

1. Create a new queue in the app dashboard
2. Configure your queue clients to subscribe to the old and new queues
3. Change your queue rule in the app dashboard to push messages into the new queue
4. Wait till the old queue has been drained completely (i.e. no messages in the queue)
5. Delete your old queue