Just curious, why are you changing the fluid?
When draining the oil, you will only be able to drain around 1/4-1/3 of the total oil capacity as a majority is held within the converter and valve body. The only way you can do a complete oil change is to drain the oil and refill then drive the car, then repeat the step at least four to five times to completely change it.
Is the box driving ok? My father has over 30 years in the business of rebuilding automatic boxes and I myself have 6 years, so if you have any queries feel free to ask.