well I can tell you how they do it----well at the shop i used to work at it may be way different now---and I think 14 is the correct number
We used to tap into the tranny lines that run to the radiator,
one is a push and one is return,
we would start the car and as the fluid was pushed out the line we would refill the tranny with new fluid.
I really have no idea why the fluids did not mix something about the specific gravity of the new vs the old
but it did not mix and when it was all said and done you have all the fluid changed
( we did usually change the filter before hooking up the lines and yes you loose like 3-4- quarts but since you are changing it anyway ti didn't matter)
and the proper Fluid is ATF +4 I think but don't worry some one will come along and correct me if I am wrong