The WiFi/Bluetooth and cellular modem are on M.2 cards, so you can replace the modem to support different LTE bands in different regions. It won’t be possible to upgrade to 5G, because requires twice as much power as 4G and more cooling than can be put on an M.2 card. Also there are new mmWave antennas that will be required in some places (like in the US) for 5G.
We have no idea if it will be possible to put a Fir PCB in Evergreen to upgrade it. Todd Weaver made a comment in this interview that makes me speculate that Purism might allow upgrading the PCB. Currently only the ShiftPhone 5.X allows the main PCB to be upgraded in a smartphone.
Really hard to know when the Evergreen batch (i.e., the mass production batch) will ship. Purism has to do FCC certification, get the cameras and video out working, and do a lot of work on power management. I’m predicting that Purism will ship in Q4 of 2020, but there will still be a lot of software issues.
These two links might help you: