Thanks ... that helped me get the crappy passphrase changed.
The process was to add the new passphrase, test it, remove the old passphrase:
cryptsetup luksAddKey /dev/sda5
That asked me for the existing passphrase which I provided and then enter my new passphrase twice. I rebooted and provided this new passphrase to make sure it worked. I rebooted once again and provided the old passphrase and that worked as well.
cryptsetup luksRemoveKey /dev/sda5
I provided my old passphrase (the one I wanted to remove) and then had to provide my 'remaining' (new) passphrase for confirmation.
Thanks for your help