Can I put cement shingles over cedar shakes?

old roof, no real leaks to speak of, but it is 30 years old. I had one roofer tell me that if the cedar shakes were in good shape, we would not have to do a complete tear off and replace with plywood. I had another tell me that no matter what the shakes were like, a complete tear off had to be done otherwise the roof would leak. Who is right?