Ah ok, I was confused by the first line of your posts in the other thread.
- Encrypted phone (LineageOS 14.1 encryption)
So this just means that the encryption was done on 14.1, but the current situation is los nightly of 15.1 (e.g. lineage-15.1-20181217-microG-FP2.zip) and you just go on with the other steps and switch/update to the latest nightly e.g. 20181220)