But... =)
I had no problems at all with my W810i before i used SEUS to update my phone.
I was just curious about this update service so I tried it. After the update the bluetooth connection with my car radio was really messed up. One 1 call was going through the car radio, all the nextcomming was only in the phone :/ Thats why I wanted to downgrade =)
I did a downgrade of the firmware but that didn't solved my problems so I did an update to the newest and that did the trick.
In my oppiniun... If you don't experience any problems with your phone, DON'T update it.
That's what I have learned.

I can add that i called both Pioneer support and SE support and SE rapair shop. SE/SE Repair shop said that it isn't possible at all to change to a older firmware but my newly experience is that it do is possible. They seems not to want to "downgrade", why is this?