Upgrading: how and when

OK. So, as I understand it, syncthings with a new minor version potentially change the protocol, so I have to upgrade all nodes at once to the new version. Is this correct? What might happen if I don’t do that (so I have a 0.7x syncthing and a 0.8x syncthing paired)? When I upgrade, do I have to set up all the pairings again, or will newer syncthings still be able to read the existing config files?

Correct, on the incompatibility v0.7 -> v0.8. When upgrading, upgrade all nodes. Nothing bad will happen if you update one and it tries to connect to the older version - they simply won’t be able to connect. All your config survives unharmed, so it’s just a question of replacing the syncthing binary and restarting syncthing.