Follow-Up:
According to the Bentley Official Factory Repair Manual for the Volkswagen Eurovan, it is only necessary to change the ATF fluid every 30,000 miles if you are driving under extreme stop and go traffic conditions, mountaineous areas, or under a heavier than normal load like pulling a trailer. Some would interpret that as relevant to the Rialta being a heavier vehicle than a standard Eurovan. So it appears that 30,000 miles would probably be a good idea to change out the ATF. I believe this is an easier process on the 5 cyclinder engines than on the VR-6 because at least you have a dipstick for checking the fluid level -- the VR-6 transmission is sealed with no dipstick. A gentleman documented this procedure on the Rialta Yahoo Group if you're interested in doing it yourself. You are also supposed to check the fluid level in the final drive part of the transmission at just about every service interval and top off if needed. The Bentley manuals state that you will never need to completely change the fluid in the final drive. If you have more specific questions, just email me directly and I'll try to help.
Here is a link that might be useful: MotorHomepage Rialta Resource