1) uninstall the old drivers. (CCC can be left installed since that is just a software tool, not the driver)
2) shut down PC and remove old GPU
3) install new GPU
4) install new drivers.
I'm fairly positive that AMD/ATI uses different drivers between the 4xxx and 5xxx series. If you were swapping say a 5570 and a 5770 you might be able to not remove the drivers, but I'd remove them anyways just to make sure everything works properly.