The iPhone 6 on release date this 2014 will lead to explosive sales for Apple as the upcoming double refresh is tipped to spur massive upgrade moves from U.S. and Chinese consumers.

Even as the tech giant is keeping most of the next iPhone details locked in secrecy, analysts are convinced that two models of the iOS-powered smartphone are scheduled for unboxing - a 4.7-inch build that will replace the 4-inch iPhone 5S and a 5.5-inch edition that is set to tussle with the likes of Galaxy Note 4 and LG G3.

It is precisely the new supersize screens that will lure new and existing Apple fans to any of the two iPhone 6 models, investment firm UBS Securities said in a new research note. Apple will surely witness a blockbuster launch for its 5S sequel, Apple Insider reported UBS as saying.

The estimate from UBS indicates that more than 50 per cent of current iPhone users would jump to the next edition following its rumoured rollout that likely will happen between late September and early October 2014.

Most of these sure iPhone 6 buyers are using the iPhone 4 and iPhone 4S, which suggests that they decided to skip the iPhone 5 and 5S to wait for the 2014 iteration, UBS said on its report.

In the United States alone, some 97 per cent of iPhone owners are eligible for upgrades by the time the iPhone 6 is out later in the year and the assumption is, according to Business Insider, a great majority of them will readily take the plunge on D-Day.

The same is true for Apple in China as the Business Insider report added that "accelerating penetration with China Mobile and strong developed country interest in the iPhone and new products may help to improve the company's fixed cost leverage."

So the bold prediction is by end of December 2014, Apple will clear around 56 million iPhones that also includes the 4-inch iPhone 5S, which will remain in the market as the company's legacy iPhone offering, said the report from UBS.

And months ahead of the iPhone 6 debut, a number of killer features for the device seem to be shaping up - one of them is the phone having rounded corners similar to the first few builds.

This was again hinted by a leaked back cover of the device that according to MacRumors was first published by Macfixit Australia. Allegedly, the back panel revealed that the iPhone 6, or a version of it, will show off a 4.7-inch screen profile in silver finish.

The highlight of the leak, however, is the seeming confirmation of previous mock ups and rumours that insist the iPhone 6 will revive the curved corners that were first seen in the initial models of the device. The rationale for the return is for the handset to accommodate a curved display panel and a slimmer profile.

Another rumoured feature for the iPhone is Split-Screen that is similar to Android's Multi-Window. In a report by BGR, simultaneous viewing of two apps appears to be certain for the iPhone 6 but it will not be previewed as expected by Apple via the 2014 WWDC this coming June or next week.

The feature will remain part of iOS 8 but will not be unveiled until the software is finalised and ready for the real world, said the same report.

That should coincide with the iPhone 6 release date that analysts have pegged for middle part of September, which specifically is for the 4.7-inch version as the 5.5-inch edition, also dubbed as the first Apple phablet, will not be out until the Christmas season of 2014.