Sounds like you should create a table for storing these values and then redefining your value list so that the "use values from a field" option lists the values from this table.
This doesn't automatically solve your issue, but it sets up a basis where an "edit value list" script can both modify the records in this table and also update records in your original table to have the new value.
And from table based value list, you also have the option of setting it up so that only an ID number is entered into your current table's field and a relationship allows you to display the text from your (now) related table. With that set up, edits to the text field in this related table will automatically appear in all records of your original table where this value was selected.
Thanks Phil. I'd had a look into something along these line but I think for the people in my office to work in this method would be asking a lot! I think the issue is when it comes to adding new values to an existing value list it might be a bit more long winded then just 'edit value list' for them.
Thanks for the advice!
Actually, with the correct scripting, it can be just as simple as "edit value list" but from a button placed on your layout instead of included in your value list. (Though that could be possible also using this value list change.)
The user clicks the button and gets a popover (FileMaker 13) or small dialog listing your value lists. They edit a value and click OK. The script then does the rest, doing any updates needed.