Press Alt+F9 to display field codes. In the field code for the date, change
DATE or TIME to CREATEDATE. Alt+F9 to toggle back and F9 to update.

There is no real alternative to editing the letters individually, however,
the following macro will convert any date field fields (fields that update
each time you open the document) to createdate fields. Add it to a toolbar
button to fix the date in the current document with a single click. At a
pinch you could rename the macro "Autoopen" and the macro would run
automatically on opening each document, but I prefer the manual approach. It
should also be possible to incorporate the technique used to batch process a
folder full of files, and there is code on my site and on the MVP site which
shows the techniques involved  and what to do with listings like this, but
I'll leave you to sort that out :)

Sub FixDates()
' Replace Date fields with CreateDate fields
' Macro created 28/09/2004 by Graham Mayor

Dim FindText  As String
Dim oStory As Range
Dim oField As Field

FindText = "^d DATE"
Application.ScreenUpdating = False
On Error GoTo oops

Documents.Add
Selection.Fields.Add Range:=Selection.Range, Type:=wdFieldEmpty, _
   PreserveFormatting:=False
Selection.TypeText Text:="CREATEDATE \@ ""dd/MM/yyyy"""
'Change the date mask above to suit local preferences
Selection.WholeStory
Selection.MoveLeft Unit:=wdCharacter, Count:=1, Extend:=wdExtend
Selection.Copy
ActiveDocument.Close SaveChanges:=wdDoNotSaveChanges
ActiveWindow.View.ShowFieldCodes = True

'Replace found text with the clipboard contents.
Selection.Find.ClearFormatting
Selection.Find.Replacement.ClearFormatting
With Selection.Find
 .Text = FindText
 .Replacement.Text = "^c"
 .Forward = True
 .Wrap = wdFindContinue
 .Format = False
 .MatchCase = False
 .MatchWholeWord = False
 .MatchWildcards = False
 .MatchSoundsLike = False
 .MatchAllWordForms = False
End With
Selection.Find.Execute replace:=wdReplaceAll

ActiveWindow.View.ShowFieldCodes = False
 For Each oStory In ActiveDocument.StoryRanges
   For Each oField In oStory.Fields
     oField.Update
   Next oField
 Next oStory
Application.ScreenUpdating = True
End
oops:
End Sub