If Ubuntu (a newer release, anyhow) is dual-booted with Windows 7 using Grub2 in
the MBR, an "upgrade" installation of Windows 8 (i. e., launched from within
Windows 7) leaves the boot loader intact. Both OS will boot after the upgrade,
as before. I found this rather surprising. You are no longer given the option of
not activating the new Windows software until 30 days after installation, so
make sure you're installing it on a computer you really want to use with it.