Thank you for the question.
Except for the prospective aspect, my understanding is that the intent of the new definition is that it would capture the same types of firearms that were in the original definition. As for the magazines, this is one element of the definition, so it can't stand alone. It's used to identify what a manufacturer is making. In looking at the magazines as a stand-alone, there are many firearms that can accept many magazines.
This is just one element to help identify a certain type of firearm.