My understanding was that any Xbox can become a dev kit and visa versa. This is from Microsoft.
Flashing your dev/test kit to retail mode merely disallows storing games on the hard disk, use of the extra memory on development kits, and use of the test tools; i.e. it makes the game act in almost all ways like a retail Xbox.
Not sure if this is something that can be done to test updates and I'm certainly not a developer, but who knows.