I'm no expert, but I have a (pre) folding stock on my Chi-com SKS.
From my understanding, your SKS has to have 3 certain items to be classified as an "assault rifle".
These include but are not limited to:
Flash suppressor
bayonet or bayonet lug
Detachable magazine of more than 10 rounds
Folding stock
pistol grip
Bipod
If your post-ban SKS has more than 3 of these, it might be in violation of section 922R (or so said a funny little BATF pamplet I picked up from my local sheriff department a few years back). I'm sure state and local laws may vary.
My Norinco SKS has an Advanced Technologies folding stock that I permanantly converted to fixed stock (using epoxy) and has the bayonet and lug removed (what good was it anyway, a mono-pod?). I am still using the fixed 10rnd mag, so I believe I am all kosher.
BATF has a toll-free number, usually listed in the front of most phone books, they may also have a web-site. Last time I checked with them, they were more than helpful to clarify the particulars.
As for items being "grandfathered in", that's something I'm not so sure about. You might want to get a definative answer on that as to not be surprised in the future, which might be a very bad thing.