The transmission fluid is the life of the transmission. A transmission fluid change or flush is preventative maintenance not a fix. A transmission has lots of moving parts the rub against each other all the time. This causes friction. Friction causes heat. Heat is the main cause of transmission trouble. Transmission fluid breaks down with heat. Different makes and models require different types of fluid and different intervals for changing or flushing the transmission. I would recommend changing or flushing your transmission fluid according to your driving habits. If you live in the city and your transmission is constantly changing gears up and down between stop lights I recommend having your transmission fluid changed or flushed every 30,000. If you live out in the country or drive long distances with out stopping you will be fine changing your fluid every 50,000 miles. If you wait and go over 100,000 miles without changing your transmission fluid, you are taking a chance when you do finally change it. Sometimes, you can do more harm than good when you go that long without changing you fluid.
Most people don't think about changing their transmission fluid until they hear a noise, the transmission shifts funny, or it quits. Then, they want change their fluid and fix the problem. Some people get lucky and a fluid change or flush will take care of the problem for a while.
A transmission fluid change or flush is preventative maintenance not a fix. Depending on your make and model, the filter may not be able to be changed. Some transmissions do not have a bottom pan and there is no access for the filter. In this case, a flush is all you can do. If there is access to the filter through the bottom pan, I recommend changing the filter and flushing the transmission.
A flush has to be done at a shop that has the equipment to flush the transmission. Most machines hook up to both of the transmission cooler lines. When the car is running, one line pumps new fluid in and the other line catches the old fluid as it comes out. This gets all the fluid out of the pan, inside the transmission, torque converter, lines and cooler. It flushes the whole system. This is the only way to change all the transmission fluid in the transmission. For a little extra, most shops will do a flush and replace the filter.
A fluid and filter change only changes about half of the fluid in the transmission. When you drop the bottom pan you only change the fluid that is in the pan, but most of the time you have access to the filter. You also get to see if there are any metal flakes, shavings or debris in the pan. If you are having problems you might want to have a look in the pan the see what it looks like.