Why do people say"If you want to get you transmission fixed, do not change the fluid?

Reading the old fluid can help the mechanic determine what is wrong. The replacement fluid will be thinner than the old contaminated fluid making clutches more likely to slip.

They say that from experience. Transmission people say it a lot about automatics. This is especially true of cars that have not had regular maintenance-not changing the trans. fluid when it was called for. Then maybe at 200,000 miles they will change the transmission fluid, and zap! their transmission falls apart.
The key I think is that if the fluid has not been changed at regular intervals per the maintenance schedule, then do not attempt a fluid drain and change-or only as a last resort before going for a rebuild. The new fluid can dislodge particles in the valve body and send them to do harm. The old fluid kept things on an even keel, more or less, and the transmission was happy with it. My take on it, anyway. Of course, this is only true of automatic transmissions, not manuals.