How I can downgrade from U3 to U1 “oreo to nougat”

I want to downgrade my phone from Oreo to Nougat My phone is Samsung J730F.

But the problem is that Samsung locked the boot-louder.

Can I go back by flashing the phone combination ROM then flash my phone with stock ROM to bypass this lock?

Note: FRP, google account protection is OFF and USB debugging is ON.

And I have rooted the phone and flash it with combination ROM.

My question is what should I do after flashing it combination ROM? Should I flash stock ROM 4 files or replace only some files?