I am undergoing the same maintenance hurdle you are facing. That is the so called myth or rule of not changing an old or neglected ATF. This adage is not a myth, but is somewhat overblown. My understanding is as follows;
- if your transmission is in good shape, then a drain and fill should be good.
- if the transmission has problems, then the new fluid might magnify them.
The true enemies of ATF are high temperatures and oxidation, and the transmission was designed to operate with good fluid. With the ATF, normally their detergent, dispersant, surfactant attributes, and the red dye are the first ones to fade over time. But its lubricating, cooling, and hydraulic attributes normally last longer.
If you add new fluid to your transmission, it is possible to lift a lot of varnish and sludge which will clutter the fluid. Those could affect its performance overwhelming the filters and reducing lubrication. Then, you need to be ready to start a short series of drain and refills, to include pan drops, magnet cleanings, filter changes if possible, and cleaning the crud in the pan. That should offset most of the adverse effects of the new fluid in the neglected transmission. Now, if it has something deeper such as weak seals, that is another story.
I would most definitely stay well clear from a flush service. I don't know of a single auto maker which endorses them.
Now, if anybody with relevant expertise is willing to expand or instruct me, please, I am listening.