I worked at toyota for four years as a mechanic and have changed many transmission fluids. In all the cases I've seen when your tranny fluid turns black you are going to gamble. Theres a chance that if you change it you wont see any difference in the performance of your transmission and things will only get worse...quicker. Things could also get better too, it's really a flip of the coin. Your transmission kind of gets used to the fulid that is in it and with all the metal shavings in the fluid, thats how your transmission learns to work. Parts selttle in places that work and they settle in places that dont work, which can cause your transmission to act up. If I was in your shoes, I would inspect the fluid by taking a small sample of it, getting it tested, and taking the results to toyota so they can make a good decision on what to do. From the sounds of it though, I wouldnt touch it. If you do, worst case, your tranny only gets worse and you end up losing it. Save some miles on it, dont change the fluid. Just my opinion. If you want to make a difference ADD an additive to it to help it perform better.