I've heard all sorts of reasons. One being that due to transmission abuse, premature failure of fluid causes the friction material to break down. But by this point the fluid is shot and has little detergent ability left. So while the damage is done, the fluid is too. Adding new fluid with working detergent may remove any remaining friction material from the now damaged bands, etc.

I dont know if thats true, but I know of two cases were a transmission fluid changes results in complete failure. Its odd.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car