Installed this upgrade from 17.x on Fairphone Open via Settings->About phone after disabling Xposed (which required a reboot). Went smoothly, though the “upgrade” downgraded my TWRP to 3.0.2 and uninstalled Xposed. TWRP downgrade not a serious problem since all I needed was at least version 3 for support of encryption (not that that seems to mean anything on the Fairphone unfortunately). Xposed reinstall was easy enough, but if you’re super serious about your Xposed, the uninstall is an issue: between the uninstall and the reinstall there’s plenty of opportunity for apps to execute uncontrolled code, export all your contacts or whatever.
I guess that these instructions are probably still the correct thing to follow if you want to ensure Xposed is in control all the time. Not entirely clear to me which of the FPOOS .zips (ota or otherwise) you need though.