The Fine Line Between IPhone Success or Failure

After more than a year of hoopla, we're finally on the doorstep of Apple's most important product event in several years. But there's a critical question that even Apple die-hards can't confidently answer: How many existing iPhone owners will buy the latest models?

This will be the most important figure to watch because the vast majority of new iPhones -- about 81 percent of the global total this year, according to Bernstein stock analyst Toni Sacconaghi -- are sold to people who already own one. Making some assumptions about the percentage of iPhone users who might buy the new arrivals, iPhone sales could increase by as little as 11 percent to as much as 45 percent in the Apple fiscal year that ends next September, according to a Bloomberg Gadfly analysis.