>Or have an option "Do you want to keep the old mbr code (y/N)?"
>(to overwrite it by default).

No.  We should *never* over-write existing MBR code, unless the user
explictly asks for that to be done.  (the only reasonable exception of
updating MBR code that we know is old NetBSD MBR code, preserving the
MBR partition table.  I dont know how to recognize any NetBSD MBR
code; does anyone?)

Someone might, for instance, have set up os-bs or bteasy to chain-boot
from a second disk.