I dropped my '67 off to have the C4 rebuilt yesterday and asked the builder this same question, after his long laugh "because everyone asks that question" he started his explanation. According to him, if you change your fluids like you should, changing your transmission fluid WILL NOT cause any harm, the problems that do appear after changing fluid are due to lack of maintenance. He said he sees it all the time, an owner will run 100K to over 200K and never change fluid, but when the tranny starts to shift funny or slip, they decide it's about time to change the fluid. Well. the problem is already there and not introduced by the new fluid, it is aggravated by new fluid. So I ask, "why does new fluid aggravate the problem"? His response was the detergent level in tranny fluid is very high, he suggested I try cleaning my greasy hands with tranny fluid once and see how good of a detergent it really is, this high level of detergent will dislodge the years and miles of crud and plug up little holes and filters quickly, making it seem like the new fluid was the problem, but the true problem is always lack of maintenance on the vehicle.