The entire purpose of this rule change is to get you to register your firearm. They don't care about the brace, they want the firearm that it's attached to.
If you know your history, then you know that there's only one reason to establish a registry.
Anything you register now is simply going to be confiscated later on. And when you register it, you're on their radar, and they have an excuse to come to your house later on.
No thanks. YMMV, but I'll take the brace off and ditch it before I invite the AFT into my life.