For all intents and purposes, its not upgradeable. Specifically, while the part in question is easily replaceable in theory, there is no market-available part to replace it with in the first place. Your best bet is a broken MBP/MBA with functioning (larger) SSD which you can salvage. Or some way to get an Apple SSD they use for repairs (no idea whether its even possible for a normal customer).
Therefore, I recommend you to access how much storage you will actually need and base your decision on that. For example, know that I won't need more then 256GB right now based on my usage patterns of the last 5 years. It might change in a few years, but I won't keep this machine for that long anyway.