This happened to me about 6K miles ago and wanted to know if this has happened to anyone else. I changed the fluid in my trans because it was pretty dark. I am the second owner of the truck and it seems like the guy before me didnt like to change it often i guess. So i change it and two weeks later my 4L60E craps out on me. It doesnt shift right, etc. I manage to get it to AAMCO and have them take a look at it. They said that it needs to be rebuilt because by putting the new trans fluid in it "changed the environment" of the transmission. He said the trans would have been fine if I left the old stuff in there because it was used to the old stuff. Now 2000$ later i have a rebuilt trans that I am happy with because it needed to be rebuilt soon anyway, but i am 2000$ poorer when i didnt necessarilly have to be. SO, has this ever happened to anyone else?