I own a transmission shop.I would be hesitant to change fluids at 180k miles of it hasn't been done before, but that's mostly because of i do it for a customer, I am on the hook.

I would drain the fluid / drop the pan and inspect before changing. If the fluid is clean and clear, but dark, change it. If the fluid is burnt, or thick, or you find metal in the pan, changing the fluid is not a good move.

By the way, Dexron is backwards compatible - so Dex VI covers Dex III. The Valvoline and Castrol multi ATF fluids are good choices.