i have a 97 chevy k1500 - 357,000 miles on the original trans and the fluid still looks good - i know if i mess with it it will fail - my vx has 115k miles and i know better than to do a flush it can only end badly - i have however drained as much fluid as i could from it and added fresh (only can do about 3 quarts at a time i believe it was) with no negative consequences yet (5k miles ago)

in my experience unless it has been flushed every 30k miles since new the flushing will almost always causes problems - the old "if it aint broke dont fix it" apparently applies bigtime to transmissions

side note: chevy 5.7l v8 = great engine