Woah! The metal backplate on the motherboard should not come off. Any backplate which is part of your existing cooler will come away when you remove it...if it doesn't, it's integral to the motherboard.
The backplate for the Kuhler 620 will line up with the holes around the CPU socket, which will be the same ones used for the cooler you currently use. The backplate you have for the Kuhler 620 should fit on the motherboard with the existing metal backplate in place.