Sub DemoReplaceBookmark()
  Dim aRng As Range, aField As Field
  Set aRng = ActiveDocument.Bookmarks("aaa").Range
  Set aField = ActiveDocument.Fields.Add(Range:=aRng, Text:="DocProperty Title")
  aRng.InsertBefore "text in front"
  aRng.End = aField.Result.End
  aRng.InsertAfter "text at end"
  'aRng.Select
  ActiveDocument.Bookmarks.Add Name:="aaa", Range:=aRng
End Sub