As the new iPhone 4S went on sale today, owners of earlier Apple smartphone models were expected to make up the bulk of buyers, with surprisingly strong interest shown by iPhone 4 owners even though some number of them have months left on their current contract.

The study, based on an online survey of 1,300 people, found that 44% of iPhone 3G/3GS owners say they will switch along with 42% of iPhone 4 owners. The survey indicates that relatively few Android users (12%) plan to switch, while about 1 in 4 BlackBerry users are enamored enough to make the switch.

The data is from the latest "Gadgetology Report" commissioned by Retrevo.com, a US consumer electronics review and shopping site. The survey was done earlier this month and graphical view of the buying plans is shown here.

Apple announced on Monday that preorders for the new phone totaled more than 1 million in the first 24 hours, compared to 600,000 for the iPhone 4 last year.

Despite being derided and dismissed by many as an unimaginative, incremental model, the newest iPhone seems destined to be the most successful ever. [See "'Incremental' iPhone 4S still promises to be big seller"]

Market intelligence analyst Horace Dediu, writing at Asymco.com, has done some thinking and number crunching and, in a post titled "How many iPhones will be upgraded next year?" suggests the following iPhone shipments, by model, through the end of September 2011:

iPhone 1: 6.1 million

iPhone 3G: 24.4 million

iPhone 3GS: 45.4 million

iPhone 4: 75.4 million

He notes that some number of iPhone 3G and 3GS models are likely to have already been upgraded to iPhone 4. He works out some calculations for the probability of upgrading the different models and comes up with about 36 million upgrades over the next 12 months, about the theoretical maximum (based on the unit sales numbers above).

Dediu has previously estimated that Apple will sell about 140 million iPhones over the "next few quarters." Of these, he now thinks, based on his analysis above, that about 36 million of those are "guaranteed" due to the desire of millions of existing iPhone owners to upgrade. He thinks Apple's track record over the past several quarters, selling about 70 million iPhones, shows that it can sell without straining too hard another 100-plus million to new customers -- to people who don't own an iPhone now or don't own a smartphone.

Apple's smartphone rivals so far lack this built-in sales leverage of an extremely satisfied and very brand-loyal customer base, Dediu argues. And several iPhone models, not just the new 4S, are going on sale with more carriers in more markets, at a greater range of prices, than ever before.

How successful the 4S will prove to be among buyers who don't own one now or who don't own a smartphone remains to be seen, of course. In the Retrevo survey, 24% of current smartphone owners say they plan to but the iPhone 4S. But 50% say they will not. About 26% say they're unsure. But these numbers don't address the much larger market of users who currently do not own a smartphone at all.

Overall, the survey found current iPhone users to be somewhat more disappointed in the 4S design and features than smartphone users overall.

Just over half of current iPhone users in the sample (53%) say "No, I am not disappointed" with the new phone. The biggest complaint, by 29%, was the lack of 4G/LTE wireless connectivity. About 1 in 5 wished for a "new design," and 12% wanted a larger display. The relative importance of 4G and a larger display, at least for this sample, is interesting: It shows a fairly significant awareness and desire for 4G, but much less interest in even a slightly larger screen.

Smartphone owners overall tracked this same distribution. Fully 71% say they are not disappointed with the 4S model. On the wish list items, 17% wanted 4G/LTE, 11% a new design and 7% a larger display. (In both groups, respondents could choose more than one answer for this section of the survey.)