[JimmXinu]

It's possible, but it's a bit involved.

There isn't a 'move' option as such, you have to copy and delete. Conceptually similar to moving files from one drive to another. Except you also want to edit it along the way.

Here's a theoretical example. I haven't tested it--I'd need a story that meets the criteria.

[archiveofourown.org]

## create a new metadata entry to edit
add_to_extra_valid_entries:,freeformchars

## copy everything in freeformtags into it.  
## NOREPL means before replace_metadata, etc
include_in_freeformchars:freeformtags.NOREPL

## copy freeformchars (after replace_metadata, etc) into characters, 
## but keep characters too
include_in_characters:characters,freeformchars

add_to_include_metadata_pre:
## include only tags from freeformchars that contain 'mentioned', 
## delete everything else
 freeformchars=~.*mentioned.*

add_to_exclude_metadata_pre:
## delete everything from characters that contains 'mentioned'.
 characters=~.*mentioned.*

add_to_replace_metadata:
## remove substring 'mentioned'
 freeformchars=>mentioned=>
## prepend Background & Cameo Characters 
 freeformchars=>^(.*)$=>Background & Cameo Characters \1

Note that, IMO, this is already broken because 'mentioned' is going to be used differently by different authors. One will use 'mentioned Victor Cachat' and another will have 'mentioned suicide ideation'.

To catch different cases like 'mentioned' vs 'Mentioned' vs 'mentions' vs 'mention:' and whether it's before or after the name would require more complex regexes and are left as an exercise for the reader. (Do text books still say that? )