well when the fluid has been in there for that long, immediately changing to a super slick synthetic atf can cause damage to the transmission. the clutches inside have, for lack of a better term, gotten used to the old fluid. the computer tends to lower the line pressure because of the older fluid and when you introduce the new fluid the line pressure is too low and the clutches get hot and you burn up the transmission. this is a really hot topic on crownvic.net. another method if you are cash strapped is to drop the pan and then refill the 4 quarts that came out. you can get away with using a cheap dexron3/mercon ATF for this kind of stuff, you can usually pick up some cheap stuff for around .99 a bottle. do that a few times then got a full flush with a filter change. when you do the full flush make sure you use a quality synthetic, like redline, amsoil, or mobil 1. no matter how expensive this sounds, its still cheaper than the 2k you will spend on a new transmission.
EDIT. something ive thought of getting, is an aftermarket traspan with a drainplug. so that when i do an oil change i can let that go and dump in a few quarts of fresh ATF. id much rather spend a little more on routine maintainence then spend a huge amount when something breaks.