You stepped on the hornet's nest. Lots of discussion on this board over the years on this one. First, the flush (power or otherwise) is a DIY procedure. See the many postings on brickboard and Bay 13. Next, this "sealed unit" story is pure BS. Volvo did not want to appear to raise the maintenance cost of the car so they originally said changing was not required.
Now, they recommend changing dirty and/or burned fluid caused by "harsh" environments. I flushed mine in several cars every 30 to 50 K miles. I have used Mobil-1 Synthetic ATF and Amsoil ATF. Once you get a good synthetic in, it seems to stay clean for a long time. I never let it get too dirty so I can't comment on the stories about changing it bringing problems on. If it's already worn out, sure, a change will not fix it.
Drain and fill is a waste of time. You are just mixing the old with the new and never really get it fresh. Only the S80s have a filter, so any wear particles are in your 'Volvo for life' if you don't flush.
The DIY flush method uses the built in pump to change the fluid, so it does not stir up anything. It is easy and can be done under the hood without lifting the car.
I'fe posted a procedure for the S80, but the 850 is very similar but a little different. It takes a 1/2" ID plastic hose about 6' long and a spare hose clip in case the old one breaks. You'll go through about 12 quarts of fluid. Should be under an hour once you have it all together. You do not need to use the volvo method of disconnecting the hose at the transmission. Disconnect the upper hose at the radiator cooler. On the 850/s70, the fluid comes out of the disconnected hose, so that's where you put the plastic hose.


That's my 2 cents.