Chosen Solution
I assume this would require some advanced soldering, but would be possible to replace the SSD with a larger capacity SSD if one was willing to radically violate the warranty and solder one onto the motherboard?
Not a chance in !&&*. Take a look: Retina MacBook 2016 Teardown
Sadly no, the density (chips on both sides and very close to one another) on the logic board makes it impossible to desolder these chips without damaging things. It would be cheaper and easier to just swap out the logic board (if you have the 256 GB model). You also won’t be able to find anything larger than 512 GB presently that fits the chips location; Orange block Toshiba TH58TFT0DFKLAVF 128 GB MLC NAND Flash shown 128 or 256 GB one on each side for a total of 256 or 512 GB. In the next year or two you’ll likely see larger density chips but again its just easier to swap out the logic board.
If it is just soldering, it can be done. However the flash memory chips are not like standalone SSD modules, since these chips are designed to be completely embedded to this board, there may be special low-level programming to be done with the new chips. So I say this is unlikely to be viable.
It might work, but you might have some problems. I’ve never tried that before with any sort of flash storage, but your welcome to.
