"..is there any means by which the passage of new federal “assault weapons” ban could result in the confiscation of firearms purchased before the passage of the bill as was done in the UK and Australia?"
Certainly, if that's the way they word the new law. The government is mighty and might makes right so far as the mighty see it.
Remember the gratest hazard to liberty the founders knew was the central government, that's why they wrote the Constitution as they did and quickly added the first 10 Amendments to limit the power and reach of government. They did NOT write the 2nd to insure we would have a right to 'deer and bird hunt' if we do it with a license and follow silly rules of the Fish and Game buffoons. Sadly, "we the people" have allowed politicians and gov. agencies, "them the mighty" to make a tatters of the Constitution, all for our own good of course.
So far as our founder's recognised Rights go, I fear we have already allowed our power mad, bloated government to regulate our rights far passed the point of no return, our kids and grandkids will never know the personal freedoms and limited government us old guys grew up with. That's gone, all for our own good of course. (Vote for a "progressive/liberal" near you, remember they only want to help us and they can't do it without our money and imposing more gov. control!)