There is a "how to change your tranny fluid" thread in the "Tech Info" thread atop this forums main page.

I use a catch bucket with volume measurement marks inside, so I know exactly how much fluid I've removed.

So when I drop the pan, catch the old fluid & measure what I've removed, I know exactly how much was removed & how much old fluid remains in the valve body, torque converter, cooler & lines.

Measuring this way, I can pour back in exactly the amount removed from the pan.

When you drop the pan to clean it & the magnet & change the filter, you only remove about 1/4 to 1/3 of the old fluid.

After refilling the pan. If you'll then remove the tranny return line at the cooler, plug it & route a length of scrap hose, from the cooler, into your measuring catch bucket, then start & stop the engine at idle, in 15 second cycles, while slowly shifting through all the gears, you'll pump out 1 qt of old fluid, each 15 seconds, verified by the measured catch bucket.

Then pour in another new qt, of make up fluid & repeat the 15 second run cycle, until you've pumped out the remainder of the systems specified capacity.

This way you won't run the tranny pump dry & will remove most of the old fluid in the valve body, torque converter & cooling lines.

Taking the tme to do this, you'll likely feel a difference in shift quality, I do.

If you can get a helper to operate the engine, it'll go faster & if open all the remaining fluid bottles, you'll probably be able to then pour the new fluid in, about as fast as the tranny pumps it out & maybe save some stop/start cycles.

On the replacement question, I think maybe you were thinking about replacing the valve body gasket????