New Lawn From Existing Lawn.
Creating a new lawn from an existing one is best done after treating the old lawn with Round Up weed killer applied two weeks before with a watering can. This will kill all the existing weeds and grass plants saving you a lot of work later when the new lawn is established,plus it will make it easier to break down the old lawn and create a new level.
After weed killing and before digging and rotovating you should consider is the soil under the existing lawn would benefit from a dressing or sharp sand to help with the soils texture and drainage.