If your transmission fluid has never been changed and it's been 100,000 miles or more then it's a good idea to change the transmission fluid and transmission filter as well.
You can sometimes get a transmission flush done but I would highly recommend changing the transmission fluid and filter.
The filter is important to the transmission to keep it working good.
I change my transmission fluid and transmission filter every 30,000 to 50,000 miles and have had transmissions last as long as 300,000 to 600,000 miles.
If you notice shifting problems early and change out the transmission fluid then it can help the shifting problems if the transmission fluid is the cause of the transmission shifting issues.
Make sure the transmission fluid is up to the proper level as well and if the slipping of the transmission gets too bad or it takes too long for the automatic transmission to shift into gears then you might need to rebuild the transmission or replace the transmission.