How often do you change the transmission fluid

Considering the high mileage > 'Strongly' recommend only draining the old fluid by removing the transmission pan, replace filter and install pan - then refill transmission to proper level using GM recommended transmission fluid!

Every 60,000 Miles or looks or smells funny . Also Filter service every other so at 120,000 miles - We use a machine that flush's 16 quarts out and install all new Synthetic . Most shops do not do this.