Comment 16 for bug 316872

update-grub wouldn't do it; it only deals with menu.lst. grub-install is not generally something that's safe to run automatically on upgrades; for one thing, it's entirely legitimate to have the grub package installed but not to have it installed in your MBR.

To be honest, this seems like a documentation issue more than anything else? If upgrading to ext4 (which already involves several manual steps), you need to rerun grub-install, and we should document that.