Ever wondered what happens to term store values used in documents when the term store entries are renamed?
Answer: SharePoint will update the values in the document using a timer job. If you want the changes to be made immediately, you can run the “Taxonomy Update Scheduler” job in Central Admin using ‘Run Now’.
If you want the old values to be retained, either add a new entry rather than renaming or capture the previous name in ‘Other Labels’. ‘Other labels’ are used to guide content contributors to the selection of a preferred term.