Modify the styles in the normal template (or some other preferred template)
then apply that template to the files with the update styles option set.
Manual formatting in the documents may cause some headaches.

Sub ApplyNormalTemplate() 'to all files in a folder
Dim myFile As String
Dim PathToUse As String
Dim myDoc As Document
Dim NormalPath
'set option to create backups
Options.CreateBackup = True
'set the location of Normal.dot
NormalPath = Chr(34) & _
Options.DefaultFilePath(wdUserTemplatesPath) & _
"\Normal.dot" & Chr(34)
With Dialogs(wdDialogCopyFile)
   If .Display <> 0 Then
       PathToUse = .Directory
   Else
       MsgBox "Cancelled by User"
       Exit Sub
   End If
End With
If Documents.Count > 0 Then
   Documents.Close SaveChanges:=wdPromptToSaveChanges
End If
If Left(PathToUse, 1) = Chr(34) Then
   PathToUse = Mid(PathToUse, 2, Len(PathToUse) - 2)
End If
myFile = Dir$(PathToUse & "*.doc")
While myFile <> ""
   Set myDoc = Documents.Open(PathToUse & myFile)
   With ActiveDocument
       .UpdateStylesOnOpen = True
       .AttachedTemplate = NormalPath
   End With
   myDoc.Close SaveChanges:=wdSaveChanges
   myFile = Dir$()
Wend
End Sub