Chuck,
you speak of the Change event but you are giving a SelectionChange
routine. With this one 1 and 1000 will be toggled whenever one clicks
on a cell containing either value.

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Value = 1 Or Target.Value = 1000 Then
Application.EnableEvents = False
If Target.Value = 1 Then
Target.Value = 1000
Else
Target.Value = 1
End If
Application.EnableEvents = True
End If
End Sub

HTH
Kostis Vezerides