Its needed for work purposes. Done alot of looking and it seems its common/allowed using Odin. I have the 8 firmware with the same bootloader version (5) and it fails to downgrade with "All threads completed. (succeed 0/fail 1). Have the drivers installed its recognized but no cigar. Im using the latest 8 release, 3-4-19 but thinking of trying the release which was used to upgrade from 7 to 8.
Just a few things to note here:
1. This could brick your device.
2. If you trip the KNOX sensor, it will make Samsung Pay, Secure Folder and pretty much anything Samsung security related no longer work.
3. A tripped KNOX sensor means that if you ever want those functions, you'll need a new phone as this could void your warranty.
4. If you do decide to do this, your updates will likely be messed up going forward, especially if you use software from a different carrier. You would have to repeat the search for, download to SD card and update process for every single update.
I need my S8+ to be on android 8 but the android 9 update got by me, is there a safe way to downgrade back to 8???
Might be taugh/not possible to go back as the new Android version most likely upgraded the bootloader which in this case you can't downgrade.