The top case is extremely easy, as the machine is built around the bottom case (assuming we're talking about the plastic model). The bottom case, on the other hand, requires you to gut the machine and rebuild it from the ground up. You'll have to pull the heatsinks off the processor, so have thermal paste handy. It's not an impossible job, but it requires a good static safe work area, a steady hand and attention to detail as there is some careful cable routing to do.