This would be a great time to switch over to a fully synthetic fluid. Mobil 1 synthetic ATF, for example

The torque converter can also be flushed by disconnecting the rearmost transmission cooler line (the return line).

hook a clear plastic hose from it to a catch basin. Put the transmission into Neutral, start the engine. Let it run until your see air bubbles in the plastic hose.

turn off the engine, add 2 quarts of fresh fluid. repeat 3-4 times until all the fluid exiting the hose is fresh fluid.

reconnect the line, and top off the fluid level.

this method flushes out all but maybe 1-2 quarts of the 7 quart capacity.

i happened to use a Quaker State 4 X 4 Fully Synthetic fluid (found it at Wal-mart). Have clean, crisp shifts in my bricks and in my '84 Olds Custom Cruiser Wagon for more than a year now.