I like to take some risks so I bought a used HU-850 + PA-300 to replace my HU-650. The swap was easy and everything works fine (sound, light up, ...).
The HU-850 come from an '06 XC70 from a scrap yard, my car is an '05 V70. Compatibility is 100%. No software needed.
This experience is similar to the RSE system (dual DVD set) wich Volvo only recommand to install in '07 and younger. I also installed these in my '05. 100% working.
So DON'T believe what Volvo say!...