Cannot change old VLAN records

[TommyC]

In Interfaces: Devices: VLAN I have a few records with names that start with "igb0_" instead of the now mandatory "vlan0" prefix.

When I try to edit the name I get an error message: "Interface is assigned and you cannot change the device name while assigned."
When I don't change the name and try to change the Parent interface I get: "The device name prefix "vlan" is required."

My goal is to change the Parent. How can I change this?

I have already tried to make a clone with a correct device name but then I get an error on the VLAN tag: "Tag for this device must be unique." 

[Patrick M. Hausen]

Unassign it first (Interfaces > Assignments).