It's technically possible. But it's not the type of modification you'd want to try yourself. You'd want to have a tech with the right skills, parts and tools to do it for you. And the replacements cost a lot more than you might expect.
-> Your problem is that the ATI and Nvidia graphics modules aren't interchangeable. You can replace an ATI card with an ATI card, but not an Nvidia card.
They're packaged on different modules.
Here is a Nvidia MXM module