Do a little at a time. On this project we added a Sony Multi-Zone AV Receiver with Yamaha Front Surround speakers and Subwoofer with a Logitech Harmony Pro to tie the old devices with the new. The Harmony Pro can trigger multiple devices at once including lighting, blinds, and the thermostat if you want.
In the Pool Room we added a second independent zone using two Yamaha Satellite speakers along with two Samsung HDTVs. The TVs were connect to the system using wireless transmitters that are capable of full 1080p and 5.1 surround sound with no latency. All powered by the same Sony AV Receiver and controlled by the Logitech Harmony Pro.