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.
#xe pool-ha-disable
Then you need to find out which host you want to change the master to. List UUID for all the hosts
#xe host-list
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
You're Done!
