The great news is the you can make the switch fairly seamlessly with the LineageOS experimental build. I’ll be using TWRP recovery, but the general process will be similar on other recovery tools. First, download the “experimental” and the latest “nightly” build of LineageOS for your specific device from the LineageOS website. Either download them straight to your Android device without extracting them or to your PC and then transfer them over to your phone. Remember that if you have CM13, you should download LineageOS 13, and if you have CM14, LineageOS 14. The experimental build is what you’ll be flashing first, then the nightly build after that.
Next, reboot your phone into recovery mode by turning it off, then holding the Power and Volume down keys while you turn it on again and select “Recovery.” In the recovery menu (This is where things might be a little different if you’re not using TWRP.), first back up your data by selecting “Backup;” then tick the boxes for Boot, System and Data; select a location to back up the data, and select “Swipe to Backup” (or the equivalent on your recovery screen).
Next, go to “Install” from the main screen of your recovery tool, navigate to the directory you placed the LineageOS RAR files in, and select the one with “experimental” in the name. Install it, then reboot your device, and your phone should load to the Android homescreen but with watermarks all over it telling you it’s a merger version. That’s a good sign, and the next step is to get rid of those pesky watermarks.
Again, boot your Android device to your phone’s custom recovery screen, then go to “Install,” and this time navigate to and select the RAR file you downloaded with “nightly” in the title. Install it using the same process as before, and reboot again.
This time your Android homescreen should boot up without the watermarks! You’ve just successfully migrated over to LineageOS. You should find that just about everything in the OS looks the same as in CM (like stock Android), except now you’ll have the perk of being able to keep your OS up to date, unlike the redundant CyanogenMod. (Just go to “Settings -> About Phone -> LineageOS updates” to set how regularly you want it to check for updates.) Note: the “Nightlies” in LineageOS actually only come around once a week.
Conclusion
It’s always a bit of a pain packing your virtual bags and moving your digital home, especially when you’ve been forced to do it by factors beyond your control. But if you’re a dedicated CM user, then you have to do it sooner or later. LineageOS is the natural choice if you want to keep things as close to a stock Android experience as possible with regular updates, but on the other hand, seeing as you have to switch anyway, maybe it’s also an idea to look around for other custom ROMs if you’re up for a change of scene. That, however, is an article for another day.
