Changing LUKS passphrase

Locate key slot

Assuming the partition holding the luks volume is /dev/sda2

As you may have more than one slot active - locate the one for the passphrase

cryptsetup open --type=luks --test-passphrase -vv /dev/sda
No usable token is available.
Enter passphrase for /dev/sda2:
Key slot 1 unlocked.
Command successful.

Add new key slot

In this case the slot is number 1 so add a new key

cryptsetup luksAddKey /dev/sda2

Wipe previous key slot

Then wipe the slot for the previous phrase (slot returned from the check) - enter the new passphrase when challenged

cryptsetup luksKillSlot /dev/sda2 1