Here's what I wrote on another thread sometime ago. The drain and refill approach on these transmissions seems to work the best. I have heard that if the TCM counter / anticipated viscosity doesn't match too well they may throw a code, but this could be heresay, not experienced it myself, as I have always gone for the full flush method / Vida reset.
As for the flushing, I first drain the ATF from the bottom of the transmission (24 or 25 mm plug as I recall), this takes out roughly 2 litres), then put in two litres of fresh ATF through the dip tube, place a good fitting tube from the top radiator ATF outlet (I use an old section of proper tube with the correct fittings and seals to avoid ATF leaks) connected via a clear polypropylene tube into a receiving container - I use a 2.5 litre clear plastic bottle, and then start engine (Transmission in park). Stop engine once air bubbles appear in the exiting dirty ATF, then refill with the equivalent volume of fresh ATF. Repeat process until only clean translucent happy ATF exits the tube. Reconnect system, warm up car properly (I would suggest a min 30 minute drive) etc, check levels for hot ATF, then top up as required. Then the VIDA reset as i mentioned earlier (possibly better to do this before the drive?). All relatively straight forward, I think that there are even youtube videos on it.
My friend was quoted $960 to do this by a Volvo dealership, no wonder the average punter doesn't get it done.........