If the old system has SeaTalk cabling, it should work...
I installed an Autohelm in our E26 last year and upgraded the chart plotter at the same time, but kept the old depth finder and the intermittent speed indicator. The new stuff comes with a relatively simple wiring approach but is (of course) proprietary and doesn't play easily with the NMEA standards. The new system is called "SeaTalk ng" and there is some good information on the Raymarine site chat rooms about mixing with older systems. If you have the old SeaTalk 1 system enabled equipment (black plugs with three female receptors) you can add that to the new system, but you will need a special adapter that converts the system over.
I have the benefit of living near Defender, and I spent quite a bit of time with the guy who works the electronics counter in the showroom. He knew his stuff regarding the systems, and he pointed me to the chat rooms and their phone technical support for the stuff he wasn't clear about. I suspect that you can replace your electronics without changing the hydraulics, especially if the older system was SeaTalk 1 compatible.
All the best,
John