While it’s always better to start fresh, it is generally safe (with most products) to install new roofing material on top of the old. In fact, some building codes will allow a second layer. When you don’t tear off the existing material, damages to the decking and roof structure can go unnoticed. There’s also a possibility that the old shingles will create bumps and imperfections in your new roof. Tear-offs are always recommended.