Automatically update document styles

How about this:

1. Attach the Template and Enable Document-Level Updating
1.    Display the Developer tab (File → Options → Customize Ribbon → check “Developer”).
2.    On the Developer tab, click Document Template.
3.    In the Templates and Add-ins dialog: 
o    Click Attach… and select your .dotx.
o    Check Automatically update document styles.
o    Click OK.
4.    Save and close the document, then reopen it.
On open, Word will overwrite every style in your document with the ones from the template.
2. Use the Organizer to Copy Styles (One-Off or Batch)
If you’d rather leave “Automatically update” off—and manually control updates, you may:
1.    Go to Developer → Document Template.
2.    Click Organizer….
3.    In the Organizer, copy styles from your template into your document (select all, click Copy).
4.    Repeat this in each document (you can automate it via a macro if you have hundreds of files).
3. Automate via VBA Macro
For mass-updates without reopening every file manually, drop this into a .dotm in your default template:

Sub SyncStylesFromTemplate()
  Dim t As Template
  ' Change path to your template
  Set t = Templates("C:\path\to\YourTemplate.dotx")
  
  ' Attach and mark for update on open
  With ActiveDocument
    .AttachedTemplate = t.FullName
    .UpdateStylesOnOpen = True
    .Save
  End With
End Sub

1.    Open each target document.
2.    Run SyncStylesFromTemplate (you can assign it to a keyboard shortcut).
3.    Close and reopen the doc to pull in every updated style from the template.