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?
How I can downgrade from U3 to U1? 


The short answer is you cannot. the u# is the bootloader version. The bootloader is protected by rollback protection fuses:

Rollback Prevention (RP) 


  Rollback Prevention blocks the device from loading or flashing an approved but old version of 
  boot components. Old versions of software may contain known vulnerabilities that 
  attackers can exploit. Rollback prevention checks the version of the bootloader and kernel 
  during both boot and updates, and blocks these processes from continuing if versions are 
  unacceptably old. The lowest acceptable version of the bootloader is stored in secure hardware 
  fuses when the device is flashed, and the lowest acceptable version of the kernel is stored in the 
  bootloader itself. Whenever a vendor-applied update occurs, the lowest acceptable version can 
  be incremented in the fuses. Because this value is kept in fuses, it cannot be decremented even 
  through physical tampering.
  Rollback Prevention fuses are set at manufacturing time in the Samsung factory to prevent old 
  firmware versions from overwriting newer ones.


This is from Samsung technical notes but it is a .pdf Download.

Flashing back to stock will not help either since it an issue with Samsung Knox securities. It will most likely end with a fatal error like: 

SW REV. CHECK FAIL(BOOTLOADER) DEVICE: 3, BINARY: 1

Mixing and matching stock firmware files maybe be possible but is an increase risk to bricking your device. If you try I would make sure to have multiple devices or multiple reports of success. Also double check to make sure if a workaround is found that it has not been patched or you have the firmware version it works for.

There might be an exploit to bypass this though I have not seen one for your specific device to downgrade oreo to nougat.

Another possibility could be a custom ROM downgrade to the android version you want that does not downgrade the bootloader.