Look at it this way, at some point the fluid has to go. My grandfather's '87 Buick Grand National never had it's fluid changed for 250,000m. It died because of it (and it has a stout tranny...).
***Flushing killed my tranny myth***
It's interesting in that this has two sides.
1) It is a myth. Flushing never killed anyone's tranny that still ahd life in it.
2) It is true. if the fluid smells burnt, has crap in it and is dark, the transmission is already on it's last leg about to die. Flushing it will accomplish this faster, but it's not going very far either way...
Just find a mechanic, or parts store that loans tools that uses a pump inlet flush machine. Those change all the fluid *after* the pan has been dropped. If you flush using a normal one, make sure the pan is dropped and the filter is changed. If the filter clogs, the fluid can not pass it, and the tranny will blow.
Just make sure of these things:
1) The tranny isn't already on it's last leg
2) *ALL* the fluid comes out of the tranny
3) The filter *HAS* to be changed
Be sure to use a good syntheic fluid. <duh>
90% AT's are killed by overheating. If you buy car, flush the fluid immediately and put on a cheap, aux. tranny cooler. It's $50-$100 to save a $3000 repair build...