Well, a "change" of fluid by just draining the case will change only about 30 - 35% of the fluid. If your fluid is brown and burnt smelling and this is all you change, you won't be doing much for it. Most of the fluid is in the torque converter which you can only get out by flushing (takes 12 - 14 quarts to do it fully--how many did the shop use?) I would look at the fluid on your dipstick and if you wipe it with a white napkin or paper towel and determine that the fluid is more brown than pink, I would definately flush it. It should be BRIGHT pink/red. Running on brown fluid means it is burnt and dirty (not uncommon on Volvo's of the year). This is your cheapest route before having it rebuilt if you find the fluid to be discolored.