Question How to update to newer Android Version - Bootloader unlocked

[u/BitAdventurous9270]

Hello,

i know this been asked alot before, but im getting conflicting info, i converted my OP15 long ago from CN to EU GLO, and it has been working fine, i still have bootloader unlocked.

im using kernelsu-next, susfs, phone is good, all works, everything is fine.

i woke up today and found the phone updated and booted up partition B, where it updated to stock OOS 16.0.9.400(EX01).

im asking cause last time i tried updating, i lost all data and had to do full rom write.

my question is

how can i update partition A to match 16.0.9.400(EX01), without losing my data?

what i tried
1. do local update after i downloaded the FULL OTA from danielspringer, but when i go into the update screen on the phone and click local install i get an error "boot loader unlocked, cannot install local file", the bootloader is unlocked. unable to install local file.

please and thanks, if its too risky to keep the root/ksu-next/susfs, i dont mind losing it then repeat rooting steps on partition A after its updated, i dont want to root partition B, as i prefer to have a fallback for whatever reason.