If they want to add animations to the face and the fingers they have to create new joints as the old rigs don't have joints there.
This means they will have to create an entire new skeleton/rig, animation rigs are very difficult to alter after they are finished it's just not worth the hassle.
The chances of something breaking are just too big, you would spend more time fixing those errors than you would just making a new rig.
For an experienced rigger that doesn't have to take that long really and you are more sure to end up with something the animators can use.
They will base the new animations on the old ones though, they won't transfer them they will make new smoother ones that look similar to the old ones.