If there is a real ban of "asault rifles" I would expect to see the lower altered so that it has a fixed 10 round mag. If you simply make the casting a little bit thicker inside the mag well it would no longer be able to accept a regular magazine. Add a hinged lower plate with the spring, throat assembley and follower and you have a top loader only rifle. Without a fast change mag the AR-15 is just a target/varmit rifle of limited use.
On the AK type rifles it would be even easier, just weld a 10 round mag in place.
Question is: Would all exsisting AK and AR rifles become class three weapons?
Sure, you would be grandfathered in BUT still subect to review before you get your tax stamp. Undeclared weapons would then become contraband. You could keep it but never be able to use it or sell it. It's value would be parts only.