Dim PreviousFormula

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
PreviousFormula = Target.Formula
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
If PreviousFormula <> Target.Formula Then
    MsgBox "Previous Formula:" & vbNewLine & PreviousFormula & vbNewLine & "New Formula:" & vbNewLine & Target.Formula, vbOKOnly, "Change detected"
End If
End Sub