This is likely an unusual question, but is there a way to change or add
a keyword phrase[1] to an existing backup image?  This is given with the
condition that I know the name of the specific image I'd be updating.

-brian.

[1] With the bpbackup command, it's possible to add a keyword phrase with
    the`-k' command line option.  It is this keyword phrase that I'm
    referencing in my question above.