So I got 2 new pc’s recently. One was a complete reinstall job, playing with the new Debian Etch installer. Unfortunately the kernel it came with was too old to support lvm at boot time, so even though I installed initramfs-tools it would not find my root partition in lvm. I had to use a rescue disk to upgrade to a newer kernel and initramfs-tools.
My other box was far more fun. I tried to move it to LVM. I made a copied the data from / into my new LVM root partiton, rebooted and found that I could not boot off LVM. Installing initramfs-tools and a newer kernel worked, but then I tried to resize my old partition smaller and make it just a boot partition, which killed grub. So a boot disk came back and I had to reinstall grub, which was full of hassle.
I do have both working now, so with a bit of effort it is possible.