I've done a lot of roofs in my day. I was always taught that you always tear off all the old shingles. It can be a lot of work. If you get a shingling fork it is not that bad. If you get any type of shingle that last for more than 10 to 15 yrs you are doing pretty good. (no matter what yr the package says they will last) The heat from having two layers of shingles can damage the ones that you are putting on now. Tear it all off, do it right and you should be fine.
