Although I haven't removed the valve body from a volvo trans, I have reprogrammed several GM Turbo350/400 transmissions. This involves dropping the pan, dropping the valve body/balls, changing out the spacer plate, replacing the governor and various springs behind pistons, etc.

Long and short of it is that it's not HARD, per se, but you do need to know (have a picture from the manual) where all the little balls are supposed to go when you put the valve body or spacer place back onto the transmission. Having a missing or very small worn ball doesn't help matters. I would call volvo and get the specific transmission model service manual for your car and review the ball placement/number. Purchase the requisite number of balls and replace them in the exact spots per the picture/diagram.

PS...get the rubber/neoprene/felcoprene or composite pan gasket. The cork gasket is a bear to install and it is easy to overtorque and it will fail in 10-20K miles, if not outright leak it will seep and is generally messy. I bought the volvo filter kit and was all ready to do the filter/pan clean drill and flush the ATF at a dealer DIY with the Washington Volvo Club and Martens Volvo in Rockville had a separate black rubber-like service gasket that they've found is really easy to install ($5 for the part) and hasn't failed in service.
Also, use BLUE (not red) loctite on the bolts and an inch-pound torque wrench!