Draining and refilling the transmission will help, but you'll have to do it several times to purge most of the old dirty fluid. There is little to be gained by dropping the pan as far as changing fluid is concerned. My Hyundai dealer just told me my car has NO transmission filter and my owner's manual seems to agree. Don't know if that's true of your Kia, but changing the filter would be the only reason to drop the pan instead of using the drain plug.
The automatic transmission system consists of the transmission, the torque converter and the cooler. In most cars, draining or dropping the pan will only remove 1/3 of the transmission fluid as most is in the torque converter with some in the cooler and lines, so you'll still have 2/3rds of the original dirty fluid. You can drain a few times to get things back to normal fluid (say every 1,000 to 3,000 miles to give the old and new fluid a chance to mix together), after 4 drainings you'll have diluted the original dirty fluid to only 19% which is pretty good.
Some garages have a transmission fluid flush machine. They disconnect the return line from the transmission cooler and connect the machine there. The machine vacuums all of the old fluid out and replaces it with new fluid. You can also do this yourself at home with no special tools. You might want to check out "transmission flush" videos on Youtube.
I paid my garage guy to do a transmission flush, but I think all he did was top it off and laugh to himself while he took my money. Maybe I should say that since I discovered this, I USED to pay my garage guy. First clue was that he charged much less than the half of cost of the fluid alone. Second clue was that he claimed to have done an oil/filter change, transmission flush, antifreeze flush, brake flush and power steering flush all in 30 minutes. Oh well, I will do this myself in the future as the Youtube videos make it seem pretty simple.
Please note that some cars require proprietary transmission fluid. I know Hyundai, Kia and I think Honda do. I've heard that Hyundai/Kia will do a chemical analysis of the transmission fluid and if it's not theirs the warrantee is voided. And their stuff is expensive. My Hyundai takes 12 quarts for a complete flush which costs $100 or more on eBay just for the fluid. I just called my local Hyundai dealer and was told that the fluid is $11.54 per quart. Yikes.
Please note my transmission experience has only been with Japanese and Korean cars. US cars may be different in the percentage of fluid removed when the pan is dropped.
Let's add to the confusion. Parts stores sell transmission filters for my car, but my owner's manual never mentions one nor does my shop manual. The owner's manual says that under normal usage I need to inspect the transmission fluid every 15,000 miles and change it at, would you believe, 105,000 miles. Under severe usage (which I think includes just about everyone) they want it changed every 30,000 miles.
