In a note to clients — as reported by “eWeek” ( — Piper Jaffray analyst Gene Munster says upgrades to the iPhone 4S could drive Apple’s overall sales numbers.

“We estimate 18.8 [million] users of the iPhone 3GS are likely to upgrade to the iPhone 4S,” he says. “Given our survey work suggests 94% of iPhone users expect to buy another iPhone, it appears Apple has built in an annuity of smartphone buyers.”

The iPhone 4S managed to presell some one million units in its first 24 hours of availability. The analyst feels the original iPhone 3GS, now free with contract, will also continue to attract users.