When most people refer to the 7's tranny as sealed, they are referring to the fact that there is no readily acessable fill hole or dipstick.
As for the fluid change, I looked it up awhile back in the TIS. The actual tranny holds something like 13 quarts of fluid, however the change volume (what you get out by opening the drain plug) is in and around 5 quarts. Less than half of the actual volume.
In my opinion if you want to actually change the tranny fluid, you will have to drain and refill, take a short drive, and repeat at least 3 total cycles before you have replaced the better part of the old fluid. Changing only 5 quarts of fluid on a system with a 13 quart capacity is like only changing 3 quarts of your oil.
To test the fluid level, you have to open the fill plug and add fluid until it starts to spill out. However, there are several tricks to the procedure, including having it at the right temperature and starting the car at the right moment to make sure its full.
IT IS CRITICAL that you use the OEM BMW filter and not any kind of aftermarket brand. Look for the name "FILTRAN" stamped on one side of the filter. If its not present, you don't have the OEM part. Using a non-OEM filter can cause the dreaded whining/whirring sounds and may damage your transmission.
It's also critical that you use ONLY the recommended transmission fluid. With a 5HP24 transmission ('97 740iL), the fluid was Esso LT... something. Your mechanic should have a chart.
Just my $0.02