As others have already pointed out, yes an Ex-Post Facto ban on "assault weapons" is possible as its been done before. The NFA did not exempt machine guns, silencers, or short-barrel rifles and shotguns (with the exception of antiques) manufactured prior to 1934 and, in order to remain legal, all of those weapons had to be registered with the Federal Government and the appropriate tax stamp paid.
That being said, the likelihood of an Ex-Post Facto Federal AWB, or any Federal AWB at all, is pretty low at this point. Even the latest AWB bill I saw that Rep. Carolyn McCarthy (D-NY) introduced (something that seems to be an annual tradition for Ms. McCarthy) did not include an Ex-Post Facto outright ban. McCarthy's legislation did allow for "assault weapons" and high-capacity magazines already owned to be kept, but it would have banned the sale or posession of newly-made examples and banned the sale or trade of previously owned weapons and magazines as well. Thankfully, even though McCarthy, Schumer, Feinstein, Boxer, and the rest of the "usual suspects" keep banging on the gun-control drum, it's become a losing issue politically and the majority of both the Democrat and Republican parties don't want anything to do with it.
Finally, while an Ex-Post Facto AWB is entirely possible, it is far from certain that such a law, even if it were to be enacted, would pass Judicial Review. Ex-Post Facto laws, historically, have not been held in particularly high esteem by SCOTUS and, particularly in light of Heller and McDonald, it seems very likely to me that the courts would strike down at least the Ex-Post Facto portion of such a law if not the whole rotten thing.
Smith, and Wesson, and Me. -H. Callahan
Well waddaya know, one buwwet weft! -E. Fudd
All bad precedents begin as justifiable measures. -J. Caesar