It appears the Switch 12 has a standard M.2 SDD and is replacable.
Mine has a 64GB LiteOn IT L8H-64V2G. With a 128GB SD Card for data I have 32.6GB free with the latest updates.
I do show the L8H series to be available in 128, 256, and 512 GB.
Note, this is the long, narrow drive and not the squarish one that replaces a SATA drive.