I’m sure this will apply to other models as well. Trying to make configuration changes to the interface gives an error similar to: “cannot edit interface, interface currently in use”. Sadly, simply unplugging the interface is not the solution. In my case, I had to remove the interface (or rather, an address that routes through that interface) as a DNS Proxy to allow it to be editable (other things I also tried that may or may not be required: deleting all policies associated with the zone the interface is in – I’ve tested this and it looks like it’s not required; deleting policy elements -> addresses for that interface; deleting an address using the interface from DNS -> Host).
I basically went through my config file looking for things referencing the zone that the interface was in / interface / IP addresses that route through that interface. Unfortunately it’s quite irritating.