2001 Volvo S80 How often the transmission oil should be cha

Hi, I have a 2001 Volvo S80, recently I had my car's trnasmission fluid changed. While the change was made, the technician at the transmission shop told me that filter change is recommened and he changed it. At the dealer they told me that is not recommended and it's easy process of graining out the old fluid. Please tell me who is right? Will the filter chenge in the transmission cause any problem to it? Will the preformance decrease? How many quarts of fluid is required? The dealer tols me 4 quatrers will be enough, is this true? thank you.