When is it too late to change automatic transmission fluid?
 

I've heard that once you reach a certain mileage, it is better to keep
the original transmission fluid instead of changing it.. Is this true?
If so, when is it "too late" to change the auto transmission fluid?
(in relation to what's best for the engine, not warranty)

I ask because I recently changed the fluid for the first time on my
78,000 mile Sonata (2000, V6). Before the change, the car was getting
stuck in gear, not shifting well, and the crank sensor code came up
during a scan. Now, it doesn't even start. I've ordered a new crank
sensor and really hope that changing the tranny fluid didn't screw up
the engine beyond the damage that was already there...

...Thanks you.