EDIT: Experimental Builds were deleted now, as initial stated by LineageOS Team in this post. CrossUpgrade is not possible anymore, best bet would be to wipe the device and start from scratch. So just skip the second step with EXP Image and WIPE EVERYTHING in TWRP and install the latest nightly. Please also stay alert on bugs which are included in LineageOS. This link will give you access to the unresolved section of the Nexus 4 / Mako device, while this link will give you access to the same for the Nexus 7 2013 Wifi / flo :). Also, you can find the latest changes for mako here and for flo here.

As CyanogenMod ended their services - and Lineage OS took over, we need to cross-upgrade to the new system.

0.) Basic idea:
To upgrade to Lineage OS, they setup an so called experimental image. This exp image exists soley for the reason of migrating old CM installations to LOS installations. An really ugly-nag-screen will remind you of that. So the upgrade path will be: CM -> LOS Exp -> LOS Nightly. Also, you need to upgrade to the correct version. I.e. LOS does supply Nexus 7 LOS 14.1 images, so you need to upgrade your Nexus 7 first to CM 14.1, then install the LOS Exp 14.1, then the LOS Nightly 14.1

1.) Nexus 4 (mako)
- Install latest TWRP Version: https://twrp.me/devices/lgnexus4.html
- Make sure your device is on CM 14.1, if not, upgrade via: https://download.cyanogenmod.org/?device=mako
- Download LOS 14.1 Exp and install via TWRP (clean caches!) and reboot: https://download.lineageos.org/mako
- Download LOS 14.1 Nightly and install via TWRP (clean caches!): https://download.lineageos.org/mako

2.) Nexus 7 2013 Wifi (flo)
- Install latest TWRP Version: https://twrp.me/devices/asusnexus72013wifi.html
- Make sure your device is on CM 14.1, if not, upgrade via: https://download.cyanogenmod.org/?device=flo
- Download LOS 14.1 Exp and install via TWRP (clean caches!) and reboot: https://download.lineageos.org/flo
- Download LOS 14.1 Nightly and install via TWRP (clean caches!): https://download.lineageos.org/flo

And thats it! 🙂

More Infos needed? Just check out this older blog entry - you can use 2) and 3) but please do not delete anything except caches on 3) - and you do not need to flash GAPPS - as we want to upgrade, not reinstall from scratch 🙂