If the atf is past the "burgundy color", I would not change. The detergents in the new atf are enacted with the heat of the trans. They can wash away varnish, loosen metal chips and grit to float in the atf and scratch surfaces or stick valves or solenoids and cause havock. From new, the atf should be changed every 25-30,000 miles with normal driving. I've always recommended changing high stalled, or cars that are "played with" twice per year as most people are really hard on their stuff. At the first sign of a problem, address the trans immediately.