How to rename volume group of an encrypted boot drive and have it still boot?

After reading George's link (and several others), I wanted to post this here as an answer.

If you need to rename the volume group of an encrypted boot drive, these are the steps you should take. At least they seem to work well on 16.04.

1. Unplug all other drives and only have a LiveUSB (or similar) and your encrypted boot drive plugged in. Note, do not boot from another encrypted boot drive because both boot drives have the same volume group name (ubuntu-vg).
2. Boot from the LiveUSB, not from the encrypted boot drive
3. After the LiveUSB is running the active system, make sure lvm2 is installed (although the 16.04.2 LiveUSB does already lvm2 installed).

sudo apt install lvm2

4. Mount the encrypted drive. Then, verify the name of the volume group (there should only be one because no other drives besides the LiveUSB should be connected). The default for 16.04 (and it seems 18.04) is "ubuntu-vg" (the following will assume that is the case).

sudo vgscan

5. Rename the volume group to something unique

sudo vgrename ubuntu-vg my_new_volume_group_name

6. You should receive confirmation that the volume group was renamed.
7. At this point, your encrypted boot drive will not boot because grub can no longer find the volume group named ubuntu-vg. So, you need to update grub. On the encrypted boot drive, go to the grub folder and see the file grub.cfg. If you try to open it from a LiveUSB, it will be read only so open a terminal and type:

sudo gedit

8. From within gedit, open the grub.cfg file (which will be on the unencrypted part, in the grub folder.
9. Save a copy of this file as backup-grub.cfg just in case something goes wrong in the next few steps.
10. Find and replace ubuntu--vg with your new volume group name (ctrl+H, Find:ubuntu--vg Replace with:my_new_volume_group_name). That find text is correct. You should find "ubuntu--vg" (notice there are two dashes between "ubuntu" and "vg" because when you use a dash here, grub needs a double-dash.
11. Save the file back to grup.cfg
12. Reboot, removing the LiveUSB

Now, when grub loads, it will see the volume group name it expects and you will be able to load your encrypted boot drive.

I actually managed to do this from a live system like this:

OLDVG=old-vg-name
NEWVG=new-vgname
vgrename $OLDVG NEWVG
OLDVG=$(echo $OLDVG | sed 's/-/--/g') # if the vg-name contains a dash, replace by double dashes as that is used for the device-name
sed -i "s/$OLDVG/$NEWVG/g" /etc/fstab
sed -i "s/$OLDVG/$NEWVG/g" /etc/initramfs-tools/conf.d/resume

/proc/mounts is used by update-initramfs and update-grub to set the root device. unfortunately, /proc/mounts does NOT get updated when changing the vg name.

therefore, we need to fake the /proc/mounts file. doing mount --bind mounts /proc/mounts does NOT work. we need to fake the whole /proc dir.

mkdir proc
find /proc -type f -or -type l -maxdepth 1 |grep -Ev '(kmsg|kcore|sysrq-trigger)'  | xargs -I {} sh -c 'echo {} && cp {} proc'
sed -i "s/$OLDVG/$NEWVG/g" proc/mounts
# we could perhaps 'unshare --mount' to create a mount namespace for the current shell. however, this did not seem neccessary.
mount --bind proc /proc
update-initramfs -u -k all
update-grub
umount /proc
rm -r proc
reboot