It's currently not possible to change the vlan ID or vlan protocol (i.e.
802.1q vs. 802.1ad) without de-configuring the interface (i.e. ifconfig
vlanX -vlandev).
Add a specific flow for this, allowing both the protocol and id (but not
parent interface) to be changed without going through the '-vlandev'
step.
Sponsored by: Rubicon Communications, LLC ("Netgate")