Transmission oil change on 2001 Accent

I had the transmission oil and filter on my 2001 Accent changed at a garage last year. It just took 3 litre of SP3. My intention is to drive around a
bit and then drain and refill. I understand that the oil will never be 100% changed but expect an improvement. Someone mentioned that a drain and refill will accomplish nothing because the new fluid will remain in the pan.
I have 2 questions.
Is a drain and refill of ATF on 2001 Accent possible without removal of the pan?
Will a drain and refill of ATF do any good? (20,000 km since last oil and filter change)
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3 liters is a little low for the amount of fluid that comes out. There should be a drain plug in the pan and another in the differential housing. If you remove each and reinstall after the trans is drained, a refill should take four to five liters.
While it cannot hurt to drain and fill the trans, 20,000km is a little soon. The service interval on transmissin originally equipped with SPII or SPIII fluid is about 165,000 to 170,000 km.
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