The process is called retrofitting and you will be much more successful with it if you have a reasonable idea of the design of your current sprinkler system. You will also be more successful if your landscape irrigation system is separated into different zones or run from different valves. If your current lawn area is on its own irrigation zone or valve, you may choose to do something as simple as removing the nozzles from the existing sprinklers and replacing them with multi-outlet emission devices. These devices allow you to run dripline from the sprinklers to specific plants in your landscape and may have room for up to 8 outlets. This is the most efficient way to irrigate your new drought-tolerant plants and there are several manufacturers of such equipment. You might also choose to irrigate your new landscape with your current system and adjust your irrigation schedule to compensate for the lower water requirements of the plants. This would be particularly effective for irrigating a drought-tolerant ground cover. Another strategy would be capping sprinkler heads that are no longer needed in your new landscape. Whichever you choose, these are all methods that you could do on your own without digging or hiring a professional.