Drop the pan, clean it out, do the full filter change, and refill it with fresh fluid. IMO the full change/exchange with a pan clean and filter change every 30-60K is the preferred way to do it and less overall effort than multiple drain-and-fills.
A drain-and-fill is like taking a glass of muddy water, pouring out 1/3 of it, filling it to the top with clean water, and drinking it. You're still drinking dirty water and the 2/3 you left in the glass contaminated the fresh water. You're better off to empty the glass, wash it out, and fill it back up with fresh water before drinking it.
A drain-and-fill is like taking a glass of muddy water, pouring out 1/3 of it, filling it to the top with clean water, and drinking it. You're still drinking dirty water and the 2/3 you left in the glass contaminated the fresh water. You're better off to empty the glass, wash it out, and fill it back up with fresh water before drinking it.
