actually, you can.
the only parts not interchangeable are the gears. I put the old clutch style sure-grip in my 489 case that originally had a cone-type. you just have to use the bearings that go with the differential unit if you swap it out. if you have 2 cone-style differentials, the bearings are the same anyway.
you could swap the cones from one to the other, but you'd be spinning your wheels (pun....heehee) because the differential halves are probably worn too. I'd swap the 2 differentials.
Alsp. I took the cone-type that I took out of the 489 case and machined the flat side down .060 to give it more life. it works, but not quite as well as a new unit.