The good book does indeed say that any new clearance given negates the previous, but you have to use common sense too. Personally when you're busy you don't have time to start re-iterating level restrictions, you'd never get any air traffic done. We also tried to get this changed so that any new clearance given does not cancel a previous restriction unless the control specifically takes it off, but ICAO threw a wobbly and told us no. Oh well so much for commen sense.