Hi Glenn, 

Our ATF LV is perfectly suitable for this application and fine to mix, Personally i recommend doing a double drain or even taking it to a auto transmission specialist as they have a specific machine to flush the old fluid out. 

When doing a double drain i drain the fluid and replace the filter (if fitted with one), refill with new oil and refit the filter, drive the car for around 500 kms or a week then do the same process again, you could repeat this as many times as you wanted to ensure you only have new fluid in the system however i personally just do this once, this will ensure there is no issues with mixing and that you have more of the new fluid in the transmission.