"Overseeding lawns" is just what it sounds like. Namely, you're sowing seed over existing grass, in order to fill in the bare patches or to thickenyour existing grass. Therefore, overseeding lawns makes sense only if the existing grass is healthy enough and abundant enough to be worth keeping. If you currently have only 50% coverage, it would be best to have us tear up the old lawn and start a new lawn from seed.