Если это простое устаревшее поле:
Public Sub ReplaceMergeFields()
On Error GoTo MyErrorHandler
Dim sourceDocument As Document
Set sourceDocument = ActiveDocument
Dim myMergeField As Field
Dim i As Long
For i = sourceDocument.Fields.Count To 1 Step -1
Set myMergeField = sourceDocument.Fields(i)
myMergeField.Select
If myMergeField.Type = wdFieldMergeField Then
Selection.FormFields.Add Range:=Selection.Range, Type:=wdFieldFormTextInput
End If
DoEvents
Next
Exit Sub
MyErrorHandler:
MsgBox "ReplaceMergeFields" & vbCrLf & vbCrLf & "Err = " & Err.Number & vbCrLf & "Description: " & Err.Description
End Sub