Change priority of a task.
CHANGE TASK changes the priority of the issuing task. It has immediate effect (unlike SET TASK), because control is relinquished during execution of the command so that the current task has to be redispatched. The redispatch does not happen until tasks that are of higher or equal priority, and that are also ready to run, are dispatched.
If you omit the PRIORITY option, the task does not lose control and the priority remains the same. This is effectively a no-op.