Depending on the version of Word, it is not always easy to redefine a hidden bookmark. In Word 2013 for instance, if I use the dialog to Insert a Bookmark, the Add button is disabled if the bookmark name begins with an underscore. In that case, I have to use VBA in the Immediate Window to (re)create the bookmark.
Code:
ActiveDocument.Bookmarks.Add Range:=Selection.Range, Name:="_Ref511073568"