You got your monies worth after using your fluid for 20 years.

Drop the pan.
If you have a converter drain plug remove it to drain the converter.
If no converter drain plug - loosen up the valve body and that will drain half the converter.

Remove both cooler lines at the trans case, blow air in to the return cooler line ( the one that attaches to the back of the main case ) with the front line placed in to a drain pan, bucket or whatever. This will remove the fluid in the lines. Inspect for debris in this fluid.

That will get out as much fluid as you can from the system.

New filter, adjust the bands if you wish.

Add ATF+4, any Dexron fluid you wish, or the preferred Type F ( used for years without issue on race cars and street cars ).

After adding about 4 quarts, with more on hand, start it in neutral and add slowly until warmed up, run it through Reverse, manual low then top it off while in neutral.