Apple formally launched iOS 7 yesterday, and hundreds of thousands of developers around the world are finally, frantically figuring out that the new mobile operating system they’ve had access to for three months in beta form will require significant changes to their apps.

And they’re not ready.

“We have companies coming to us in an absolute panic,” Matt Johnston, the CMO of uTest, told me this morning. “You are going to read some horror stories of very popular apps two weeks after launch that suddenly see their app reviews go way, way down.”

Update: Apple just announced its iOS 7 ‘Tech Talks’

The differences between iOS 6 and iOS7 are both subtle and obvious.

iOS 7

Above: iOS 7

Image Credit: Apple

While hopeful developers can try to simply recompile their apps for the Apple’s new mobile operating system, iOS 7’s new look and feel means your old user interface might look clunky and out of place. Worse, apps that try this cheapskate method of upward compatibility are seeing significantly more crashes on iOS 7, Johnston says, and any user of the iOS 7 beta over the last three months can confirm (me included).

More significantly, the new operating system renders elements differently, with overlays and transparencies, and old apps could find elements not just looking odd, but also actually out of place. For example, iOS 7 has OS-wide text size settings, meaning that some buttons with dynamic text in your app could be faced with overflowing text. And one key challenge is the curveball Apple threw everyone: 64 bit.

“A lot of companies weren’t expecting this … it caught everyone off guard,” Johnston says. “That means that the world of development and testing just got that much harder — now you have a 32-bit iOS compilation and a 64-bit iOS version.”

Not all developers have been entirely surprised, of course.

Many have been planning for iOS 7 since summer, working with the beta release, and gradually adjusting their apps to an iOS 7-optimized version. They’ll be ready to launch as soon as Apple pulls the trigger on the OS — and Apple did notify all iOS developers yesterday, asking them to submit their iOS 7 apps as soon as possible.

But while uTest, which tests web and mobile apps for customers like Google, HBO, and Amazon through a global crowdsourced army of 100,000 testers in 200 countries, saw a significant number of requests for iOS testing in July, the number jumped 10 times between the first week of August and the last — and even higher in last couple of weeks.

iOS 7 apple“The number of inquires blew away anything we saw in August,” Johnston told me.

The new iPhones are actually great for owners, he said. The iPhone 5S, particularly, with its high-powered 64-bit processor and beefy graphics capability, is going to provide for significantly better and more engaging apps, especially games. But they do bring new challenges for already-busy developers, with a change that rivals that from the first iPhone to the second.

And the easy way out isn’t really the easy way out:

“iOS 7 is a different user experience, and it is a significant difference,” he said. “If developers just compile for iOS 7, there are going to be noticeable flaws.”

In difficulty lies opportunity, however, and some companies are using the switchover as a chance to differentiate themselves from the app store pack — and make more money:

“For some developers, this iOS change represents opportunity,” says Todd Anglin of cross-platform development tool maker Telerik. “A mini gold rush may occur as developers hope to ride the wave of interest in new iOS 7 ready apps. Some developers, who have for years been largely forced to give away iOS app updates for free, are using the major iOS 7 transition to introduce new versions of their apps that must be re-purchased.”

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