https://twitter.com/iFixit/status/1186349702578466817

This will be interesting.
From my own research so far:
Internally there is a M.2 2230 M-Key NVMe SSD ( model used: (Toshiba)/Kioxia BG4 ) or (depending on the model, maybe random) a M.2 2230 B-M-Key NVMe SSD ( model used: SK Hynix BC501: https://www.skhynix.com/datasheet/eng/popup_link/popups/BC501_(M.2_2230_S3)_popup.jsp ):
Source 1
Source 2
Source 3
Source 4
Looking at the spec sheets of BG4 vs. BC501, BG4 clearly wins (also PCIe x4 vs. x2 Controller).

Looking at retail availability of M.2 2230, here in Germany you can currently buy the Western Digital PC SN520 (~90€ for 512GB) and Toshiba BG3 Client (~120€ for 512GB) and this review says a retail version of the Kioxia BG4 will be released in Q4 2019.

The maximum size you can upgrade to with M.2 2230 is 512GB (previous generation) and 1TB (current generation).

They appear to be using normal Torx screws:
https://i.imgur.com/GSnorbl.png (Source 5)
Also the Alcantara models have a removable top as well (look to the right):
https://i.imgur.com/F2Wc7sm.png (Source 6)

The question remaining is how easy you can remove the rubber feet to get to the torx screw and whether the SSD has some Apple like hardware protection "magic".