You can plant and grow new grass on an old lawn if you follow the right steps. To accomplish this you will need to patch and overseed the lawn. Patching will help even out your old lawn by sowing seed of the same type that is currently there. Overseeding will help the grass on the old lawn by augmenting it with a new type of grass seed. The aim is that this new seed will eventually become the dominant grass without the need for you to dig up your remaining old grass. It will also remain visible on your lawn year round.
Rake the existing lawn thoroughly so any bare patches are visible. Pull up any loose patches of grass with the rake. Dispose of any leaves or grass patches as compost. Go over the spots to be seeded twice, once horizontally and once vertically. Removing all weeds and vegetation gives the best result.
Cover the lawn with compost to fertilise the new grass seed. Cover the entire lawn area with no more than 2.5 cm of compost. Save enough compost to sprinkle on the seed after it's sowed.
Water the composted lawn in sprayer fashion. Wet the grass just enough to moisten the soil. Wait 15 minutes before the next step.
Use your hands to scatter the grass seed over the entire lawn. Make sure you don't step on the new seed. Don't over-seed to avoid thatching. A rule of thumb for old lawns is to use roughly half the amount of seed suggested on the bag.
Sprinkle the leftover compost on your grass seed. Cover with a standard grass canvas, which will allow for the proper amount of sunlight.
Water your fresh seed once more with a hose sprayer. Repeat this every day that it doesn't rain to avoid dryness. Wait until the new grass blades are a few centimetres high before fertilising normally. Mow the new lawn once it's 7.5 cm high.