You can modify the failover capability of a messaging engine during runtime. However you should not do this unless it is absolutely necessary.
You modify the failover capability of messaging engines by changing the policy that applies to the corresponding high availability group. The servers in the cluster will receive the configuration update and the changes that you make take effect as soon as you click Save. For example, if you have a One of N policy which has no preferred servers, and you change it to enable fail back and add a preferred server, when you click Save the HAManager will ensure that the messaging engine is running on the new preferred server; if the messaging engine is running on a different server, the HAManager will move it to the preferred server.