Depending on the car, there is either a drain plug in the transmission pan, or the pan can be dropped (add the cost of a new pan gasket to the procedure), or the mechanic can hook a flushing machine to the transmission cooler lines. I think 30,000 miles is a pretty short interval to change the transmission fluid, although many here swear by it. But at 120K it is definitely time to do it, if you plan on keeping the car.