1. It will never happen any time in the near future, so it's pointless to think about how to prepare for something where the circumstances could be radically different, especially when you consider how many other happenings are far more likely to occur. Basic, general preparations for SHTF will cover your ass for the initial period in 90% of situations, including this one.
2. If it did, you should know what to do with them already. The you-know-whos wouldn't want me saying it here so I'll share with you a thought experiment. Let's just say that you may consider taking up fishing and gardening as a hobby when the buybacks start. You may consider taking your guns with you to do some hunting while you still can, but remember to strap them down so they don't fall into an ocean, river, or deep lake where you can't recover them. If by some terrible accident it did happen, report them being lost to the police. If the police ask about the freshly dug hole in your yard, remind them that you've taken up gardening as well. If they ask you what you've been doing hanging around the newly formed fishing and gardening club in your neighborhood, tell them it's the hot new thing for outdoorsmen now that guns are banned. Of course nothing illicit has occured, just good, clean fun with some friends for whom unfortunate accidents seem to be fairly common.