You can use the following commands to change the Citrix XenServer pool’s master server:

First you need to disable high availibility:

xe pool-ha-disable

Then you need to find out which host you want to change the master to:

xe host-list

Then change the master:

xe pool-designate-new-master host-uuid=[uuid of new master host]

Turn HA back on:

xe pool-ha-enable