It is sealed in the sense that there is no dipstick so no "easy" way to change the fluid like older non-sealed transmissions. But in reality isn't actually sealed as it has a drain plug and a fill plug.

I did a transmission fluid change on my previous FJ Cruiser (same engine & transmission). I used the method of disconnecting the return line from the radiator to pump out 2 quarts at a time. There is a procedure for filling it to the correct level at the proper operating temperature. Overall I thought the job was surprisingly easy. More steps than an engine oil change but about the same level of difficulty.

As for change versus flush, I think by flush they mean using a machine that flushes out the old fluid under pressure. I have always heard that is a bad way to do it. A change I think means replacing the fluid using at most the transmission's own pumping action, or just draining and replacing the fluid in the pan. The method I used with my FJC just uses the transmission pump. Some shops will have fluid swap machines that operate by the same method.