I had this question come to me recently, and I had to do some research about this because steel roofing is still not a common practice. True, there are parts of the country where it is more common than in other parts, and it is becoming something of a trendy thing to do. But who has ever heard of laying steel over old composite...
These are not precise instructions for how to do this; rather an overview of how the process might be done by you or a professional. Also, keep in mind that many cities are not so enlightened about the "trendiness" of steel roofing, instead of hearkening back to the day when steel roofing meant only auto shops and backyard sheds.
The composite shingle roofing can stay in place. In fact, with many roofing applications, the existing shingles can stay in place and do provide an extra measure of protection against the elements. The only factors which may prevent this are the added weight of all of this new roofing, as well as the difficulty of laying new roofing onto often mangled and rippled asphalt shingles.
Wood Framework Base for the Metal
But the way that we get around it with the steel roofing is to construct a wood framework of one-by-four's and then lay the metal on top of that framework.
It is very much like constructing a non-loadbearing interior wall framework and sticking that framework on top of your roof. And like an interior wall, the interstitial studs of your roofing framework are 16 inches on center, too.
This framework effectively bridges and masks any imperfections in the existing composite shingles. Also, you can purchase rough-sawn one-by-fours, which will be much cheaper than better grade lumber. It does not matter because the bottom of the framework will have contact with the shingles, and the other side will only have contact with the metal.
What do you do with the exterior-facing parts of that framework? You can apply fascia board, and paint it just like you would any other exterior trim.
This is just one way to apply metal roofing over composite shingles. A professional roofer or contractor may give you different advice based on the condition of your house.