I can't speak for the Volvo, as I've only had one now for 3 months. But for our other cars, a Honda and a Nissan, I've changed them out before there was CEL indicating oxygen sensor failure. I know some people will say they don't need to be changed until a CEL comes on, but I think the upstream sensor should at least be changed, as the upstream one usually controls the fuel mixture, while the downstream only monitors the CAT for failure. On the Nissan, I changed it at 75K and on the Honda at 100K. Yes, the cars ran better and got better fuel mileage or at least back to normal. I only changed the upstream sensors, on those cars.
Thinking about doing in our 06 2.5, but haven't found the location of the sensor yet. Not sure if its a pain to do.
You may want to try the dealer service manager to see if they will give you a free one since it is part of the emission system. It may be worth the call, to see what they say. I think the emission system has to be covered for 10 years per EPA/CARB.