That was the plan, drain and fill, then do it again in short order until the fluid is clean(er)
It’s far better than doing nothing. However you didn’t say the condition of the fluid. Unless it’s red, I’d do a filter and then 3-5 spill n fills in succession. Literally drive it tot the store or for a day and do it again. Defeats the purpose to leave most of the old fluid in for a long time.
Same with brakes and power steering. Brakes I wait until the fluid is good and mixed. You can tell by the color change. Power steering, same as trans. Suck the res dry, refill, drive around the block, repeat until the old looks almost like the new.
This obviously uses 2-4x as much fluid but with most of the benefit and FAR less effort.
A good vacuum extractor is a very handy inexpensive tool to have for anyone who services their own stuff.
I went 20 years doing it the hard way. Not sure why….
Coolant, axles and tcase Id do as well but obviously just once.