Found the Answer:
If you already have CM or an unofficial AOSP based ROM, you don't have to wipe your device - in the download section there's an "experimental" build that you can install over your existing AOSP based ROM and it will migrate you to LOS without needing to wipe (potentially - it may not work and boot loop, in which case you'd need to wipe anyway).
Download both the nightly/weekly/stable version of LOS 14.1 and the "experimental" image.
Boot to TWRP
Install the "experimental" build
After a reboot, the system should load and a watermark saying "cm to lineage migration build" should be all over the screen. If this is the case - everything worked fine! Continue to the next step. If it didn't boot, just reboot to TWRP and wipe the device as per the instructions above and continue with the instructions in the article.
If everything went fine, reboot, go back to TWRP, install the regular image and reboot back to a fully working Lineage OS.
However, now I need to find experimental build which was removed from LineageOS download section couple of months ago.