Retrofit snow guards are for installing on existing slate roofs. You do not have to remove the slates or shingles in order to install these snow guards! There are two general types: ones with notched shanks that slide up underneath the shingle and hook on the shingle nail (Berger and Gough), and ones with a hooked top that slide over the head of the slates (Mullane or Salvo).
Note: the notched retrofit snow guards have a "reach," which is the maximum distance they will slide under a slate and still hook a nail. Measure your slate from the bottom edge to the nail hole in order to determine what reach is required for your particular slate. Some imported slates have high holes and some longer slates (22 inches long and 24 inches long) may be too long for the snow guards you're looking at. If your reach is too high, use Mullane or Salvo retrofit snowguards! Or custom order Gough retrofit snowguards with extra shank length.