Hi there,
For what it's worth, I'm in the same boat as you with that same issue.
Through long hours spent with GTAC, it turns out there's a bug reported for this back in November that hasn't been addressed yet.
You are unable to fully remove a device and corresponding VIQ from an XIQ account in order to add it to another XIQ account.
Even if the VIQ is deleted, you would get the following error when trying to add the device serial to a new XIQ account.