Transmission flushes are a huge profit center for many shops. They usually remove a cooling line pump in the old and out with the new. Would you change the oil in your engine without changing the filter? I doubt it so why not change the filter in the tranny. This is a messy job but one that can be done easily. If you have a pump with a small enough hose your can pump out some of the fluid through the filler tube, if not it is going to be messy to remove the pan but then that is what floor dry is for. You can remove most of the fluid from the tranny by disconnecting one of the cooler lines and pumping out the fluid. Some techs will add a couple quarts of fluid as it is pumping to run some clean fluid through the system. I also add a container of lube guard when changing fluid, this will darken your fluid some but this is not a problem. I don’t think there is a tried and true change interval as this depends on the abuse the tranny gets. If you pull a trailer or 4X4 a lot then change it more often. Check your owner’s manual for the proper fluid and don’t take an aftermarket counter persons word that this is a one fits all fluid. I have fixed many transmission problems by installing the correct fluid. This is a major problem with Chrysler products and many rice burners. As far as synthetic goes I am not sold on it in automatic transmissions just too many variables. However it is great for manual transmissions, differentials and transfer cases.