As egas and Billy stated. The DIY 2 qt incremental drain is a common means to gradually exchange the old fluid out and with very good success rates. In worst cases, dropping the pan and cleaning the shift solenoids. If the car is still shifting the fluid exchange usually takes care of it. The fresh fluid has detergents that clean so if the fluid is more on the blackish side, the 2 quarts gradually introduces itself. A full fluid exchange is fine on trannies that have received regular fluid changes as the accumulation of particulates and residues are far lower than a neglected tranny.
For DIY, Type IV from the Toyota dealer is sub $5 but may be as high as $7. Also get the metal crush washer, these are single use only and cost about $1 but I've seen them as high as $3.50.
What comes out goes back in so measure carefully. The tranny performs nicer with the fluid at proper levels. To much or to little can have shifting issues but are easily remedied.