Shoulders work fine, they are only ever so slightly smaller. Those with a rim around them are at least as big though if not bigger.
Heads work better on new sculpts than old ones if anything.
Backpacks also work, the difference is negligible.
Weapons fit perfectly from old to new.
Torsos work but will require some work, these are more noticeably smaller. Once you add the rest though it's quite passable.
You also can't do half and half, as the new ones are big enough to cause misalignments. But again, works, requires greenstuffing or some other way of hiding the faults.
Arms are too thin and short, at least those from FW, haven't tried with old GW arms yet.
Legs are likewise much shorter and thinner and don't really work.
That on the picture is a FW torso and shoulder. With the terminator head from the new sculpts.
I had to pad the arms as it was too narrow and it's still a bit awkward, might have to add some belt or whatever to hide the armpit.