Right now, I'm worried that he may look at New York, see that as a big win, and do the same basic thing as an EO. Why? What's he got to lose. If he does impoase a ban, and says "turn them in by 1/31 or face prosecution" the people would have only two options to challenge the EO.
Option #1...get arrested and convicted for violating the EO. Appeal. Lose the appeal, Appeal to SCOTUS. The process could take years, and you may sit in prison while the wheels of justice inch along.
Option #2...a gun owner files suit in Federal Court seeking a declaritory judgement that the EO is unconstitutional. Process takes months. Say Owner wins, Feds will appeal. More months go by. Say Owner wins the appeal, Feds will appeal to SCOTUS. All the Feds would need to do is get one justice to either find that (1) a mag that holds more than 7 rounds is not in the common use, or (2) Heller was wrong. Even if SCOTUS says EO is out because of Heller, we're two + years down the road, and Obama is getting ready to hit the public speaking circuit.
Meanwhile, we all have to choose...obey and give up our mags, or become criminals/rebels.