I know this post is old, but the answer still relevant.
In VSX, the sync interface must be a dedicated interface (including bundles), and change_interfaces only supports moving one physical interface or bundle to another.
If you want redundancy for your sync interface in VSX you must dedicate a new bundle for that purpose... but in R80.40 I have found both Sync and Mgmt are not available in vsx_util change_interfaces. I am not sure why; I am testing a migration where the lab hardware doesn't have Sync and Mgmt and I'm guessing the reconfigure will fail for this reason. That may be an issue as sk86121 (change physical interface names) does not apply to bundles (bonding).