I'd been following the Gentoo Wiki guidance on upgrading Grub, and had been taking it very carefully. I'd worried about getting this right, as getting it wrong would leave me with a brick, so I'd been very pleased to see the notes on using the old bootloader to chain load the new one. That way I could check that my configuration was correct before taking the plunge of installing the new version into the Master Boot Record. I didn't want to automatically generate the new config file, as I didn't trust it. (Rightly so as it turned out, because my initrd files didn't follow the strict naming requirements, so weren't picked up by the config generation script) Anyway - the hand-written config was half a dozen lines long, and the generated one was utterly incomprehensible.

So anyway - I managed to create the config file, and get everything set up for chain loading. I rebooted the server, and bingo - there was the chain loader entry in my "old" boot screen, and when I followed it, I got the new menu and could boot the server. Great stuff! Now it should have been a simple question of running grub2-install, and I'd be finished. So I did this, and then.... the computer wouldn't start. Fortunately I had a grub prompt, so grub was "working" - but it obviously couldn't find its config file. I already knew that with the right incantations it might be possible to get the thing to boot without a config file, and after a bit of googling, I got enough clues to attempt it. (For the record, what I think I'd done wrong was to fail to remount /boot after my chain test and before running grub2-install, with the result that grub then didn't know how to correctly find /boot.)

It took a few attempts, but the command line completion in grub helps a lot. This is what I eventually ended up typing at the grub prompt to get a working boot.

grub > set root=(hd0,1)
grub > linux /kernel-gen-newudev-3.3.8-gentoo root=/dev/sda3
grub > initrd /initramfs-gen-newudev-3.3.8-gentoo
grub > boot

Note that the root for the boot loader is different from the root of the operating system, so you have to specify them separately. Obviously YMMV for the names of the kernel and initrd files, not to mention device identifiers.

But the real advice here is to avoid missing out that crucial mount operation!!