The simple, if sorta incomplete answer, is that the CONSTITUTION of the UNITED STATES provides that there shall be no retroactive laws passed.
That is why when the first ban was passed that we had 2 classes of weapons, you cannot outlaw that which was already legal at the time that it was made. Go figure that the best and brightest legal minds worked up the first ban. If there was a CONSTITUTIONALLY legal way that they could have gotten rid of all 'Assault Weapons" during the first ban, don't you think that they would have done it?
Hence, if another ban is passed, there will again be 3 classes of weapons, Pre ban, NO BAN but unmodified, and Post 2nd ban. There will be, naturally, a provision that if a weapon was not modified up to NO BAN standards during the no ban time frame, that it cannot then be updated AFTER the passage of a SECOND BAN.
Short answer, modify it now, and they can never take it from you unless they outlaw the gun completely.