This is because SW was deployed as soft fork, therefor an opt-in feature. The implementation of SW in Armory intents to respect that disposition. Think of it this way: the software will not lead to coins in opt-in/new script types without some guarantee that there was deliberate user intent.
...as it should be
I had run the arguments successfully when I changed the received address type - I must've restarted armory without them before trying to change the change address type. Doh.
- and thank you achow101 for the 'merits' - who knew something I knew nothing about would make me smile!