As long as you're just replacing the O-ring, you can do it on the car without too much trouble. It is tight down there but you don't need too much room to pop the cover off. I ended up cutting a wooden dowel and lodged it in between the cover and the transmission tunnel to keep the cover depressed while prying out the clip.
Visually check the B2 piston while it is in there, as long as it isn't in pieces you're probably okay for now. If it is shifting ok then you are probably fine as well, but can't hurt to take a mirror and peer in there to make sure. If you were to replace this, you'd need to loosen some mounts and the driveshaft support bearing to shift the tranny over a few inches.
Dieselgiant has a pretty good explanation of the o-ring replacement process, exactly the same problem that you are experiencing:
http://www.dieselgiant.com/Mercedesb2pistonsealreplacementl.htm
You won't have to pull the piston out.