It is possible to reroof over old shingles, with some precautions taken for the existing roof shingles and the asphalt base. The asphalt base needs to be in good shape, and the shingles should lie flat with no edges curled up.
Care needs to be taken that all leaks or potential leaks have been repaired in order to help assure a successful reroofing over old shingles. Special flashing that has a special drip edge is designed specifically for reroofing and gives it a tight seal and a longer wear. Having the right tools and a helper is also helpful for the project.