Usually the only way to get the old filter seal out is with a pick....which can in turn score the bore the filter seals to. Only change the seal if the old one comes out with the filter. I'd change the filter, not just the fluid.

I would use Dextron VI, not III, despite the higher expense. If you have the OEM deep (stepped) pan, you'll need about 5.5-5.7 quarts with a filter change. The OEM shallow pan (flat bottom, which you probably don't have) takes a little more than 4 quarts in most cases.

You might have to remove the trans mount nut from the rear crossmember and raise the trans slightly in the rear to be able to get the pan off. Also, it's usually much easier to take out the two torx 40 bolts securing the shift cable bracket than to fight it (some people remove the cable from the bracket then bend the bracket of out the way).