you had LineageOS installed already though?
Yes, indeed and twrp, on my OnePlus3. New installs require complete wiping afaik.
Depends on your device state. If you've unlocked your bootloader already, you should only need to wipe system-level partitions (which would affect applications, but not user storage).
But if you haven't, you need to do that first, which requires you to wipe your entire device, including data.
Edit: Well for anyone reading this, I just did this and everything works and I have android 10 (with all apps and their data intact. Took 10 min, very nice).