Remember just because you can’t see it doesn’t mean there isn’t a problem.
We do not recommend any type of shingle to shingle layover, It is always best to tear off your existing roofing materials in order to inspect your decking, chimney, side walls and any other roof penetration areas properly. If you fail to completely tear off your roof the consequences could be very damaging to your home. It would be such a disappointment to get a brand new roof installed, then have to tear off shingles due to a bad inspection of your roof decking.
There are many roofing contractors that believe it to be “OK” to leave the exiting felt paper they might tell you “its better roof protection” or “better insulated” the fact is these are lies. Felt paper is made to protect the decking on your home therefore, you do not need to have 2 or more layers of felt paper on your roof. Many brands offer different types of felt paper, some even have a synthetic underlayment that we also recommend.
To give you a better idea here is a comparison of the 15 lb and 30 lb for the CertainTeed Brand.