If you are adding/removing interfaces in ClusterXL, the way to avoid a spurious failover due to interface "failure" is to cphastop the standby, complete all your changes on both cluster members and the SmartConsole, install policy to both members, then cphastart the standby. See this rather old SK for the detailed failover-free procedure:
sk57100: Adding or removing an interface in ClusterXL High Availability topology might cause fail-ov...
Since you will be modifying the sync interface, as an additional precaution you may want to uncheck "drop of out state TCP" in the Global Properties ahead of time and reinstall policy, on the off-chance an unexpected failover occurs when state sync is not working. Having this box unchecked will blunt the unwanted effects of a non-stateful failover; just don't forget to recheck it when the work is complete and tested!
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com