Why whould a fluid change damage your transmission?
If you pull the stick, and it does not look like Cherry Cool-Aid, Change it!
These myths about "gunk" and "flakes" are bunk. The failure came from, (say it with me) "Lack Of Maintainence" There are stories about this all over the internet.
It's CRAP! If your trans fails after a fluid change, it was going to fail anyway!
New fluid is the best thing your trans can hope to have in it! The lubricating and hydraulic properties of trans fluid are damaged by heat! If the fluid is overheated, ie. over 250 deg. F it is probably junk. If your trans has a plugged filter, it probably needs to be rebuilt.
ATF, of any spec, Dextron III, Mercon, ATF+4, or MerconV, or ??? and the list goes on. They all are prone to heat failure. There is no reason to think that changing the fluid on your already abused trans is going to cause a failure.
When you drop the pan, and the magnet is "fuzzy" You might have a problem.
Otherwise, buy a new gasket and filter and fill it back up. Keep in mind that the torque converter will hold about 70% of the fluid. So if you can't drain it, change more often.
It would be the same if you were to say the same thing about an engine. "don't change the oil, "the damage has already been done".
No wonder I'm still fixing these things,
Change The Flippin' Oil, no matter what it's in
Damn, I'm Good!
"I've got dozzens of friends, and the fun never ends, that is, as long as I'm buying."