If it's like the last one (it probably won't be -- it will be much more restrictive and maybe even have some teeth) the following will hold true ...
You don't have to turn anything in. You can keep your semi-auto rifles and sell them to someone else, you can keep your "larger than 10 round" mags and sell them to others ... you can even continue to buy new most types of rifles, so long as a few cosmetic items have been removed. You just can't buy those things new (unless altered as described) after the ban goes into affect (throughout the 10 years of the last ban, prices went up on banned items but there was NOTHING that's wasn't easy to find).
I don't think our government will ever find the support or the political will to do a massive gun confiscation. And if they do it will cause many problems.
The way the eventual gun ban will work (if the anti's win their long term plan, and Heller decision is too weak to help us) is that one kind of gun at a time will be banned or restricted, affecting only new guns, making sure that existing guns get more expensive and the population gets used to not being able to buy firearms. Through attrition legal guns will slowly cease to be, as every gun stolen and then seized back by the police will become an "illegal gun" and not be returned to the population.
So if an AWB looks imminent, stock up on whatever is being banned.