gpilot
n3wb
While I can't help (yet) what you are dealing with is that newer firmwares past 2019, maybe 2020, are signed. I am told by a reliable source that the partition structure may be changed.I have a successful connection via UART and TFTP, but each "run" commands ends up with an error message: "[ERR0002:]The img header be changed!"
Have compared the BL, and have seen that the old version was:
"U-Boot 2010.06-svn5354 (Jan 31 2018 - 21:02:02)"
now the new has:
U-Boot 2010.06-svn6470 (Dec 11 2018 - 16:27:39)
I am in the same position as you. Downgrade attempts (I used serial and tftp to flash each file in the extracted firmware.bin) result in the error you received.
I did not attempt to flash the bootloader, however I now believe this to be necessary. When you flash firmware with the new bootloader I believe it is verifying the signature of some or part of the partition header. So its doubtful to me whether you can downgrade without getting access to the flash chip directly. I don't know (yet) how to disable those checks or if there is some way access to the flash rom from the PCb(s). and not flashing through the existing bootloader. Hopefully I am wrong.
If you succeed please tell!! If I find a way I'll post here.
Good luck