Not just Elements, but AWD vehicles in general should have all tires replaced at once. The reason is a worn tire is slightly smaller in diameter and circumference than a new tire. So if you have two worn tires and two new tires on the same car, it's almost like having two different size tires. Different size tires spin at different RPMs at a given speed. If you always have your front wheels spinning at a slightly different RPM than the rear tires, it can damage the AWD system. All four wheels should more or less always spin at the same speed, except for very short periods of time, such as when you're going around a corner.