@graff
it's possible to do the u-boot update/downgrade via serial terminal, connectiong a null modem cable to a PC, settings 115200 8N1.
In order to do the update from the serial terminal, under AmigaOS4.1, you need to enter before the following commands from the Shell:
nvsetenv stdin serial
nvsetenv stdout serial
and then reboot.
From this point, when back to u-boot, you can stop the boot sequence pressing 'q' key at the countdown, then enter the commands to upgrade/downgrade u-boot via TFTP.
When finished, reboot to AmigaOS4.1 and from Shell enter:
nvsetenv stdin usbkbd
nvsetenv stdout vga
to restore the original settings.
Hope this helps