New trans fluid has detergents in it that "could" loosen up the crudge(technical term) in a "seasoned" trans. This "crudge" can get into areas including the vb that cause all kinds of havoc. The slipperyness(also a technical term) of the new fluid can also cause clutch materials to, well, slip. This slipping isn't good. The additives can also cause seals to shrink or expand letting pressure by and clutches to not be applied as needed.
I'm not saying not to change fluid and filter, just that with high mile units can suffer. If'n it ain't broke, fixing it until it is can result.