I haven't heard anyone post a definitive answer, but I think that's because there isn't one. I can tell you my opinion, which is that it doesn't matter all that much why they changed the recommended interval. Life expectancy isn't a single number, so that the chances of a belt lasting six years is 100 percent and lasting six years and one day is 0 percent. Rather, it's a range of probabilities. If you don't hit 90K miles, the chances of the timing belt failing after six years is extremely low, and the chances of the timing belt failing after seven years is still extremely low (although it's probably
ever so slightly higher than after six years). However, this probability increases over time, and at an increasing rate. We've recently heard two reports of failed original timing belts on cars that haven't hit the mileage interval. So the real question becomes, what's your comfort level? If you want to be absolutely as safe as possible, change it after six years or less. If you want to be almost as safe, you could wait until you hit seven years. But each additional year increases the chances that your unchanged belt will fail, which can be very, very expensive. Maybe you think you can go seven, eight, even nine years or more, and it's your car and your decision whether or not to do so... but at some interval you will be kicking yourself afterwards if the belt fails and you face an expensive repair. I recommend changing it at least six months before you reach that interval.