Transmission Fluid Change - Dexron-III to Dexron-VI

I had some questions on doing a transmission fluid change in my '04 Impala (81k miles). The car was bought at an auction and had no maintenance records that came with it, so I have no idea what kind of ATF fluid was used or when it was changed last time. I'm assuming that the current ATF fluid is Dexron-III and I was planning on using Dexron-VI. I already have a new filter, a new transmission pan gasket, and about 16 quarts of Dexron-VI ATF. I know that dropping the pan will only get about 7 to 7.5 quarts of fluid out of the transmission, which holds 10 quarts or so.

Is there a fitting on the transmission cooler for a line through which I can drain the transmission fluid or should I just remove the transmission cooler line? My plan was to do either one of the above, turn on the engine, drain about 2 quarts into a bottle, turn off engine, add 2 quarts of Dexron-VI. Repeat this about 3 to 4 times. Then drop the pan and clean it, change the filter, reinstall the pan with a new gasket, add 8 quarts of Dexron-VI. Would this be the correct procedure for replacing the ATF with the newer Dexron-VI?

Would it be easier (compared to the above procedure) to just drop the pan (replacing filter, gasket, and ATF), drive around for 15-20 minutes, drop the pan again, and replace the fluid one more time?

Also, there is something in the Haynes manual about replacing the old filter seal with a new one after removing the filter? Would the local car part store carry a filter seal? I've been looking around the rockauto website for it, but I'm not quite sure which one it is...

I've never done a transmission fluid change, so any help would be really appreciated. Thanks!

All you need to do is drop the pan and change the filter. The seal your refering to is in the kit with the filter, usually in a bag or sometimes put right on the filter tube..
Make sure the old seal comes out as sometimgs they will stay in the hole the filter tube pushes into.
As for the adding fluid and re-changing it, I see no point in doing that but if your going to want all the fluid clean, go to a trans fluid change center, have them do a flush, they dont do filters, go home, drop the pan, replace the filter.
You can take your fluid to these places and they will install what you are using.
there is nothing wrong with the dextron 3

My plan was to do either one of the above, turn on the engine, drain about 2 quarts into a bottle, turn off engine, add 2 quarts of Dexron-VI.
if you replace filter, clean pan, and add what fluid you removed , which was in pan about 4qts , then with engine still off remove transmission cooler line to radiator start engine and as 2quarts comes out add 2 quarts repeat until clean, or untill you have replaced 10quarts this should get all old fluid out safely , so that no particles will mess up the control valves in tranny. this is the repair manual fluid replacing proceedure. then with engine at proper temp. top off to operating area on stick if too much fluid suck out to proper level when hot.....