Replace Field Contents can do a "batch update" of your records.
Perform a find for all records that have the value you want to remove.
Edit the field of one record to give it the value that you want all of the records to have.
Leave the cursor in the field and use Replace Field Contents to update all the other records to match.
Since Replace Field Contents can modify massive numbers of records and cannot be undone, save a back up copy of your file before trying to do this.
Thanks. I'd considered that as well, and it's good to hear a pro suggest that method as a first response.
I suppose I was hoping that a triggered script would enter the conversation as well. So if a user were to modify a Value List, they could then re-validate the field of records to make sure all values adhere to the Value List.
But this did the job... ;)
You never asked for a script.
Replace Field contents is also a script step. But there's no script trigger that is tripped by modifying the custom values defined for a value list.
You might modify your value list to be a field based value list with a button used to edit the records that are used as a source of values for your value list. You might then be able to set up a scripted method that is an automatic part of the value list editing workflow.