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Here is another discussion about the topic that clearly shows the transmission is in a bad shape already and may soon break down whether you change the fluid or not. Again, it's like contemplating an operation on a person with fears of complication. Don't operate and just "wait" or do and hope for the best. Bold emphasis is mine.
to quote:
Yes, if you haven't serviced it on a regular basis, doing so now could actually cause it to start slipping. Especially if you flush it.
If the transmission is already damaged, those metal shavings and other debris is providing the friction to keep it working. If you flush that fluid out and put in new fluid, there might not be enough friction to work anymore.

Draining it and refilling it might not be as bad though. It depends on the amount of damage. But if you drain it and check out the fluid, you will get a better idea. Since just draining it doesn't remove all of the fluid, if there is damage, the old fluid that still remaining in the converter might be enough to keep it working. Kind of like a fish tank, you leave a little of the old water in there for the algae and bacteria the fish need. The inside of the transmission is worn and damaged, it needs the friction at this point.

But if there is damage, at one point or another you will need to replace or rebuild the transmission. It wont matter how much friction you have if everything is broken and worn down to nothing. But replacing the fluid really isn't going to help at this point, as far as keeping the transmission in good shape. That's why a transmission service is called "preventive" maintenance.

And no, the transmission doesn't get "used" to the old fluid. New fluid is better for the transmission and will prolong the life, however if the transmission is already damaged, the friction provided by the old fluid (and all the debris in it) is what is keeping it from slipping, for now. It will eventually completely tear up the tranny and it will need to be fixed. But changing the fluid at this point could very well cause it to start slipping.

The best thing would have been proper maintenance in the first place... We can all agree to that. At this point, I think your best option is draining out some of the old fluid and adding new. Flushing would completely remove all of the old fluid, so just drain it instead. You will be able to inspect the old fluid for debris and see how bad it is.

Contact any reputable tranny shop and ask them if they will flush your transmission fluid. Then tell them you have 150k + miles and have never serviced it before. 99% of them will tell you no, and why. Then again, some will tell you sure, because they know you will be forced into getting a rebuild right after the service.

http://answers.yahoo.com/question/in...8141032AAiyHOW