Versione ricorsiva senza troppi fronzoli nè controlli.
Ti consiglio di testare la macro su una copia della tua cartella. Devi solo modificare la riga del percorso.


codice:
Sub trovaSostituisciRicorsivo()
Set fs = Application.FileSearch
Dim doc As Document
Dim wd As New Word.Application

With fs
    .LookIn = "C:\Documents and Settings\pippo\Desktop\pippo1\"
    .SearchSubFolders = True
    .FileName = "*.doc"
    If .Execute() > 0 Then
        For i = 1 To .FoundFiles.Count
            'MsgBox .FoundFiles(i)
            Set doc = wd.Documents.Open(.FoundFiles(i))

            wd.Selection.Find.ClearFormatting
            wd.Selection.Find.Replacement.ClearFormatting
            With wd.Selection.Find
                .Text = "casa"
                .Replacement.Text = "moto"
                .Forward = True
                .Wrap = wdFindContinue
                .Format = False
                .MatchCase = False
                .MatchWholeWord = True
            End With
            wd.Selection.Find.Execute Replace:=wdReplaceAll

            doc.Save
            doc.Close
        Next i
    Else
        MsgBox "Nessun file trovato"
    End If
End With

wd.Quit

End Sub