FP 4 from Lineage OS back to FP OS with closed Bootloader

I want to go from Lineage back to FP OS with closed bootloader. I understood the whole manual, except for the following important sentence at the end:
Quote: “If you install an OS with an older security patch level than your previous OS, Android’s roll-back protection might brick your device when locking the bootloader! Wait until you get a software update with the same or newer security patch level before locking the bootloader.”
My question: At this late stage, the previous operating system that was on it is already history, isn’t it? Another question: Does it only depend on the security patch level, or also on the Android version, because that would be a downgrade from 13 to 12.

Greetings Martin

Well… if you moved to Lineage OS 20 then you are out of luck!

Fairphone OS is “a bit” behind the latest security patches and Android have some protections in the bootloader to prevent doing… “weird” stuff…

So as long as you can’t (or we) replace the bootloader with an older one… you’re stuck with Lineage OS until Fairphone OS get updated…

I’m so sorry…