So from an MX into a GTX.
Or would you have to disassemble the entire laptop, which would not be worthwhile in the end?
No, that's not possible with most laptops. Add to that, that you (if you could exchange them) only on Ebay Classifieds and Co. At a not cheap price. Is not worth it.
Most laptops have a built-in graphics unit that can't be replaced, but there are models that do not, and have a dedicated graphics card that can be replaced, which could make other changes to the hardware necessary
Arrives on the laptop.
In some laptops, the graphics card is soldered, so you can't replace them without the corresponding devices.
Also, the space is very limited in a laptop, so you have to look if the space available for the other graphics card is sufficient.