I just had the same situation in a home that I am remodeling. The whole wall behind the kitchen counter was formica that matched the countertop. Like your situation the backsplash was installed before the top and bottom cabinets. In my situation I was also replacing the countertop, but the formica went behind the base cabinets. A good sharp chisel cutting the formica around all of the cabinets did the job. Don't worry about the plaster/drywall behind the formica, you can patch it since you ( like I did) will be installing ceramic tile as the backsplash. Looks terrific.