I mean this is the home screen:
You can dirty-flash 18.1 + MindTheGapps over 17.1 + OpenGApps.
Originally I had the same concerns as you but this has worked for me and other users.
On 17.1 I was using the OpenGApps nano package since back then it was advised to not use any other package. Not sure what will happen, if you are using another OpenGApps package.
Are you coming from official LineageOS 17.1? In that case dirty flashing worked for me and it has kept my apps, settings and data.
Thankx for this information. I used ADB Sideload for upgrading to LineageOS 18.1. Afterwards I installed Gapps and cleared Dalvik and Cache. My device booted properly and everything is still there. Looks like everything is working properly.Are you coming from official LineageOS 17.1? In that case dirty flashing worked for me and it has kept my apps, settings and data.
Still you should backup all your important stuff before the upgrade.
Regarding formatting: I don't remember anymore what I did precisely, but I'm pretty sure I did not format any partitions. If anything, then I have wiped (not formatted) the "Dalvik / Art Cache" and "Cache" after installation of the ROM and the gapps. But this might not even be required.
Here is the official guide on how to upgrade to a higher version of LineageOS (e.g. lineage-15.1 -> lineage-16.0): https://wiki.lineageos.org/devices/polaris/upgrade
There it does not say anything about wiping/formatting.
Do you see anything similar to the logcat in https://gitlab.com/LineageOS/issues/android/-/issues/3851
Do you have TWRP or OrangeFox recovery installed?
adb rootshould not be required now
adb devices -l
You will not get it from https://download.lineageos.org/polaris anymore. You will have to download it from some 3rd party sites where you can not be sure whether it is free of malware.