Is it possible to change the full disk encryption password after installation?
Open up a terminal and type:
sudo cryptsetup luksChangeKey /dev/sda5 -S 0
If you want to add another passphrase:
sudo cryptsetup luksAddKey /dev/sda5
If you want to see which keyslots are enabled:
sudo cryptsetup luksDump /dev/sda5
If you want to erase keyslot #1:
sudo cryptsetup luksRemoveKey /dev/sda5 -S 1
DO NOT erase the last remaining keyslot without creating another one or you'll lock yourself out of the disk.
Hat tip to the good ole Arch Wiki