I'm trying to upgrade an OpenSUSE Leap 15.4 system to 15.5, and after many
tries, I'm getting nowhere. In a previous bug report (1223167), I failed
because I couldn't burn a working DVD and was told to use USB instead.

Now I'm trying with USB, and I have a new problem:

Booting
Cannot upgrade the bootloader because of a mismatch of the boot technology. The
upgraded system uses Legacy BIOS boot while the installation medium has been
booted using EFI boot.
This scenario is not supported, the upgraded system may not boot or the upgrade
process can fail later.

ERROR: No proposal

I created the USB 15.5 image for upgrade using SUSE Studio Imagewriter. The
current system normally boots off of an NVME drive and was originally installed
with OpenSUSE Leap 15.4 using all of the defaults. I'm using an Asrock X99
motherboard.

After numerous searches on the web, I found no real answers for this, but lots
of invective and just nasty replies.

It's somewhat shocking that the installer can't just live with the existing
boot mechanism when it's being operated in "upgrade" mode.

Apparently, the "fix" is to somehow convince the BIOS to boot from USB in
legacy mode. There's just no option for that, though. The only selection is
UEFI. All of the "legacy" things in the BIOS are turned ON, including the CSM
legacy support. Looking at the USB image in fdisk, I can see that it has an EFI
System Partition on it.

Is there some way to force the ISO image itself to be MBR-based rather than
EFI?

It seems like I'm not the only person with this problem. I just don't see a way
out other than starting over with a different distribution.