There are some laptops out there that allow you to change the GPU and CPU, however they are expensive. Probably so, that it will be cheaper to just buy a new laptop
I think you are probably better of with getting a good laptop now, something with a i7 6700HQ and possibly 960m/970m. That should hold out for the next few years. But I would not buy that laptop right now, new GPUs are coming out in June/Juli from AMD/Nvidia. Nothing has been said yet about laptop GPUs but I imagine there could be a few new ones also out around fall.