Tracking down BTX behavior change.

I have a problem on my system where 11.3 boots fine, but any supported
release (11.4, 12.2, 13.0) fail during the BTX stage. There's a bug
report if you're interested[1]

But my question is: how can I track down differences in BTX code? I
have the 11.3 and 11.4 SVN sources checked out, and so far I am not
able to find any differences.

So far, I am looking in: src/stand/i386/btx/ and src/usr.sbin/btxld

I am on amd64, and I do not see any /amd64/ directory, so I am just
guessing that /i386/ is what I want.

Also: given that the 11.3 boot code works fine, I wonder: what are the
ramifications of using it on an 11.4 system?

I am not really experienced with the boot loader process, so am not
sure what the dangers involved are.
Certainly, I am able to boot in to the new (11.4) kernel with the old
userland installed. But my woes happened when I did 'make
installworld', which presumably updated the boot code.

Would it be reasonable to manually roll back the boot code to 11.3,
but keep the 11.4 userland? Again, I'm really not sure the
ramifications, just trying to make my system usable again.

Thanks
-John