With its price target at risk of falling below the company's actual trading share price, Canaccord Genuity on Thursday updated its forecast for Apple stock to $660, and revealed the results of a new survey that found the iPhone 5s is the most popular handset at all four major U.S. carriers.

Analyst Michael Walkley's latest update came in a note to investors, a copy of which was provided to AppleInsider. It comes on the heels of a recent surge in Apple's stock price, which temporarily pushed it above $600 earlier this week.

Apple's high of $604.41 put it very close to Walkley's previous price target of $610. Still, other analysts remain "underwater" with their projections, and have not provided further revisions to their targets following Apple's recent gains.

Walkley, however, justified his new price target of $660 as his firm's surveys conducted in April found that the iPhone 5s was the top selling smartphone not only at all "tier-one" U.S. carriers, but also at a number of global channels.

The April surveys also found that customers are increasingly opting for early upgrade programs, and are showing a greater propensity toward more expensive smartphones when enrolling in such programs. All four major U.S. carriers now offer such programs: AT&T Next, Verizon Edge, T-Mobile Jump, and Sprint One Up.

Walkley sees strong adoption of these programs as a good sign for Apple, and in particular the company's anticipated "iPhone 6" launch later this year. He believes that those upgrade programs — along with anticipated new, larger screen sizes for Apple's next-generation iPhone — could result in an especially strong upgrade cycle for the Cupertino, Calif., company.

In addition, Walkley believes that the carrier upgrade programs could allow Apple to charge a premium for larger-screen iPhones. Rumors have suggested Apple will launch a so-called "iPhone 6" in two screen sizes later this year: 4.7 inches and 5.5 inches.

"We believe Apple could charge a premium for a potential phablet-type iPhone 6 SKU, and with carrier subsidies or smartphone leasing-type payments to generate solid incremental gross margin dollars from this device, even if it partially cannibalizes some iPad tablet sales," Walkley wrote.