You can change it yourself, drop the pan, and measure the amount of fluid you pull out. adding the same amount of fluid back in. To check fluid level, you need a special tool.
The fluid should be changed no more than 50 to 60k. 100k is for cost of ownership and nothing to do with reality.
If you have a NAG1 do a pan drain and fill. Before you do so, get a zip tie and measure throught the filler tube what the level is hot and then also when it is stone cold. This way you don't have to get the special dipstick. Drain the oil and clean the magnet. Replace the filter and gasket and use permatex. Then pour the fluid in till you get to the cold level of measurement. Run the car in neutral for 5 to 10 minutes and then shift back and forth from reverse to drive.
Drive the car and then look at the level. Add any as necessary so that you get it to the hot level and you are good.
Make a loop by putting the Zip Tie through the holder so you have a "ring" at the end to hold on to.
Put it down the dipstick tube when stone cold and measure it marking it with a sharpie marker.
Then when the transmission is to operating temperature, put it down again and mark it hot.
you then have the differences between hot and cold. This will give you a reference when you are ready to change the fluid - Pan Drain and Fill Procedure.