Insidious Calm, +1--ya beat me to it.
I have little doubt that there will be some number (I hope a large one, but who can know) of LEOs, Federal agents of various types and military personnel who would refuse any such order.
I know from personal experience that the "unlawful order" training in the US Army (circa 1982), while explaining that we had the right and duty to refuse an unlawful order, also made it extremely clear that you had better be right if you ever refused an order. Who determines if you're right? The officers on the courts martial panel. Sell the officer corps on the action, and you'll get most, if not the vast majority of, EMs and NCOs.
Waco, Ruby Ridge, Elian Gonzales, the ATF and LEOS in Richmond, etc. etc., etc. We know that has happened. All we're really discussing at this point is scale.
The wildcard is the reaction of individual gun owners. A lot, probably most, will cave and deliver their guns--look at Austraila and the UK. Many will keep some of them (in the US, the ones bought "without paper") and be quiet. They'll get caught, a few each week. Some, maybe just a very few, will let the authorities have their guns "bullets first"--Molon Labe and all that.
If those few cause enough damage, they may pull in some fence sitters and some of those who haven't been caught with their non-papered guns. And then they're going to be reading a certain book we all know about for tips.
Of course, the easiest way to keep this discussion is to be dag-gone sure that the order never gets issued in the first place. Vote, be politically aware and active, and so on.
Prayer is probably also in order.