It depends on how well the documents were formatted to start with.
Work with copies!
Try, in the document that is set the way you want, going to the Design tab and saving a Quick Style Set.
Then, open a copy of the one you want to update and use your custom style set.
Otherwise, here is information about the
Organizer.
[Edit: The method using the Style Set will not work unless the TOC styles are in the Quick Styles Gallery in the source document. They are not there by default.]
The following macro will convert all TOC styles to be Quick Styles so they would be saved in a Quick Style Set.
Code:
Sub TOCStyles2QuickStyles()
' Charles Kenyon 2018-02-19
'
' TOCStyles2QuickStyles Macro
' Set all TOC styles as QuickStyles so they can be included in a StyleSet
' Also makes these automatically update to match formatting
' This is preparatory to saving in a Quick Style Set
'
Dim i As Long
'
For i = 1 To 9
With ActiveDocument.Styles("TOC " & i)
.QuickStyle = True
.AutomaticallyUpdate = True
End With
Next i
End Sub
[Further edit: 2/20]
Upon further thought, try it without running the macro. The TOC styles are based on Normal which is included in a normal style set. They may well work in the new document because of changes to the normal style. Again, it all depends on how well styles were used in the two documents.