You don't have to change the XenServer pool master, but in a situation where you need to do a maintenance on the pool master you will have to move the pool master to another host.
First you need to disable high availability.
Then you need to find out which host you want to change the master to. List UUID for all the hosts
Then Change pool master
# xe pool-designate-new-master host-uuid=(uuid of ne master host)
Turn HA back on
# xe pool-ha-enable
If the pool master is not available the switch has to be enforced to succeed.
# xe pool-emergency-transition-to-master
Re-establish connection to the slave hosts
# xe pool-recover-slave