Metal roofing over shingles isn't a problem as long as the roof can take the additional load (not likely a problem). You may need to trim some of the shingles if they are curled/cupped really badly, otherwise just fasten down some strapping and use long screws to go through the metal/strapping/shingles/decking and get well into the roof rafters. If you're paying to get it done, the main difference is cost. Removing shingles is a pain in the butt and generates a good deal of waste material to be disposed of. Contractors charge accordingly.