If you are uncomfortable removing the drain plug for whatever reason, your approach will suffice as long as you add fluid and then turn on the heater and let it run for a few minutes and then add more fluid until it is very difficult to tell the difference in color of what is coming out and what you are putting in. The primary reason to change fluid is that over time the fluid becomes corrosive so removing the vast majority of the old fluid solves that issue.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas