The proper way to update bookmark content is to add the content to the bookmark, not after it. Use code like:
Code:
Sub UpdateBookmark (wdDoc as Document, BmkNm as string, NewTxt as string)
Dim BmkRng as Word.Range
With wdDoc
If.Bookmarks.Exists(BmkNm) Then
Set BmkRng =.Bookmarks(BmkNm).Range
BmkRng.Text = NewTxt
.Bookmarks.Add BmkNm, BmkRng
End if
End With
Set BmkRng = Nothing
End Sub