laptops with discrete GPUs usually have an integrated heatpipe solution that cools the CPU, GPU, and chipset. at least that's been the case with all the ones I've opened up. so you would probably have to replace the GPU also. even without the GPU, the CPU/RAM/mobo is already a pretty big portion of the BOM.

at that point, I have to wonder how much you would actually save by not replacing the screen, chassis, and storage. overall, this concept strikes me as more of a win for repairs than upgrades.