Can my MacBook Pro A1990 storage be upgraded?

If you look at Step 3 in this MacBook Pro 15" Touch Bar 2019 Teardown the four Toshiba TSB3226AW8815TWNA1 and TSB3226XZ2939TWNA1 flash storage (256 GB total storage in this given system) marked in Blue.

While it is technically possible to replace these chips with larger ones this takes a bit more than just swapping the chips alone, they need to be programmed with the correct code and you do need the proper skills and tools to do it.

Apple offered custom builds with the followed setups: 1 TB, 2 TB, or 4 TB for reference.

I think you’ll find buying a used system is a cheaper and safer way, I wouldn’t worry about the external physical condition as you could just swap out the logic board with its mated TouchID/Power Button with your systems logic board or just get a newer system which meets your needs.