I think the general rule that not to change it unless you're having trouble isn't the best, and not the first time I've heard it.
Personally I'd say if you're having problems it's probably to late!
BMW Automatics a case in point, they were sealed! No drain plug fitted.
Sorry, but no oil lasts forever
Mines had it's changed every 4 years since it was new, so it's another job I need to add to the sunny-day list