I wold not know the difficulty of doing this to the Nexus, but I know from expierience that upgrading the flash storage is not fun in an iphone, so you can guess how easy it will be in other devices, I would inspect the tear down, and decide if your ready to go as deep into the nexus as the internal storage. If you have the skills anything is possible, just buy the right part and DON'T MESS UP.
Have fun (: