I think you'll find the HU803 you've bought is pre-facelift (2004/5) whereas your HU850 is post-facelift so they are different shapes and mounting types. As the units are quite different I'd be surprised if the PCB was interchangeable.
If you end up going for an aftermarket stereo, I've got a fascia plate and all the wiring needed to fit a double DIN unit for £40.
