Even with "flushings" there is some of the old oil left it the convertor and other tranny internal cavities ... it's just the nature of mixing. The only way to completely rid yourself of the old oil is to disassemble/rebuild the tranny, emptying and refillilng every component.
With "machine flushings" ... the ones where external equipment is used to pump "cleaners" and then "new fluid" thru the tranny ... you get not only old oil left behind but cleaner fluid, and who knows what else, left in your tranny. This process is a cash-cow for Ford's (independent) dealerships and other service centers ... and has ruined many trannys. Ford (corp) has a battle going on with their dealerships over this issue. Ford gets stuck with an irrate customer who wants his tranny repaired on warranty and the dealer who wants to act innocent of wrong doing. You will not find this procedure recommended in any Ford literature ... or any other auto manufacturers literture, I suspect.
"Self flushings" are a different matter. This process involves using the tranny's own oil pump to pump fluid thru the tranny. The tranny fluid is pumped into a connected waste container until the pump cavitates ... extra fluid is then added, several extra quarts usually, ... and then the fluid is pumped out until the pump cavitates again. The external hook-up is removed and the tranny fluid topped off to normal level. This procedure is especially effective if after the first pump cavitation, the oil pan is dropped and cleaned and the filter replaced. This procedure you will find in the Ford shop manual for many models.
Instead of flushing, many folks get along just fine using the old "drain and refill" process. Tho this obviously leaves much of the old fluid in the tranny ... if it is done often enough, the fluid will be fairly "fresh". Once again, adding the dropping/cleaning of the pan and replacing the filter makes the process better.
BTW, I doubt if your tranny has a torque convertor drain plug ... but I don't know.
Here is what AutoZone has to say:
http://www.autozone.com/servlet/UiBr...3d800b8925.jsp