The plans themselves have changed many times before. They become obsolete over time. What if your plan ID becomes unavailble, or the specs change in a way your application can't handle? Don't shrug off your bad design decisions on someone else.
Deprecated plan ID requests should return two plan ids (new_price <= old_price and new_specs >= old_specs). Then a bit of retry logic means the code will still function sanely until the hard coded ID can be updated (or it may continue to function indefinitely).
