One thing to take note when dealing with old transmission fluid is that it contains a amount of clutch material suspended in it.
Which can help worn parts within the transmission to operate.
The major trouble happens in many cases that when you dump and replace a large amount of fluid those worn parts do not function as they once did.
Which in a lot of cases cause shifting issues.
That is why I said if your going to attempt a fluid change, do it slowly over the course of time one quart at a time.