The code you posted looks problematic as you could delete content all over the document if it gets a mention in oldPN. In my opinion, bookmarks are too unwieldy to use nowdays - especially where there are more than one instance you need to find - you should be using Content Controls instead.
For instance, if the CCs had a title of PNBox you could update the content in each with
Code:
Sub CCupdate()
Dim aCC As ContentControl
For Each aCC In ActiveDocument.SelectContentControlsByTitle("PNBox")
aCC.Range.Text = "hello"
Next aCC
End Sub