Find/Replace MS Word with Mail Merge Fields

The following code isn't a complete solution, but it might get you moving in the right direction.  
This is a VB program that opens a document and replaces merge fields.
(You need to add a reference to Microsoft Word Object Library.)

You could modify this example by passing in the document to open, and a collection old and new fields.

Private Sub Command1_Click()
    Dim word As word.Application
    Dim field As String
    Dim lastpos As Long
       
    Set word = New word.Application
    With word
        .Visible = True
        .Documents.Open """C:\Documents and Settings\TestField.doc""", False, False, False, "", ""
        .ActiveDocument.Activate
   
        .Selection.WholeStory
        .Selection.Fields.ToggleShowCodes
        .Selection.Fields.ToggleShowCodes
        .Selection.HomeKey Unit:=wdStory
        .Selection.GoTo What:=wdGoToField, Which:=wdGoToNext, Count:=1, Name:="MERGEFIELD"
        .Selection.MoveRight Unit:=wdCharacter, Count:=1, Extend:=wdExtend
        If Asc(Mid$(.Selection.Text, 1, 1)) <> 171 Then
            .Selection.HomeKey Unit:=wdStory
            .Selection.WholeStory
            .Selection.Fields.ToggleShowCodes
        End If
        .Selection.HomeKey Unit:=wdStory
        .Selection.GoTo What:=wdGoToField, Which:=wdGoToNext, Count:=1, Name:="MERGEFIELD"
        .Selection.GoTo What:=wdGoToField, Which:=wdGoToPrevious, Count:=1, Name:="MERGEFIELD"
        .Selection.MoveRight Unit:=wdCharacter, Count:=1, Extend:=wdExtend
        lastpos = .Selection.Start
        Do
            'Do Replace
            field = .Selection.Text
            If Asc(field) <> 13 Then
                field = Mid$(field, 2)
                field = Mid$(field, 1, Len(field) - 1)
                               
                Select Case field
                    Case "City"
                       .ActiveDocument.MailMerge.Fields.Add Range:=.Selection.Range, Name:="New_City"
                    Case "Last_Name"
                       .ActiveDocument.MailMerge.Fields.Add Range:=.Selection.Range, Name:="New_Last_Name"
                End Select
            End If
            lastpos = .Selection.Start
            .Selection.MoveRight Unit:=wdCharacter, Count:=1
            .Selection.GoTo What:=wdGoToField, Which:=wdGoToNext, Count:=1, Name:="MERGEFIELD"
            .Selection.MoveRight Unit:=wdCharacter, Count:=1, Extend:=wdExtend
        Loop While lastpos <> .Selection.Start
        .ActiveDocument.Save
    End With
End Sub