Ehh.. clarification. In many cases the GPU is changeable... If it is dedicated, and there are GPUs available that use a similar socket.
But an important thing to note is that the upgrade is costly, and normally not worth it.. and sometimes might require a little DIY modding on the heatsink.
If you head over to the ROG or NBR forums you'll find a few people who've been able to cram a 6970m into laptops that would normally carry a 5870m... etc So it is def. possible, just a lot of work on your part.