Apple Inc. CEO Tim Cook said when debuting iOS 7 in June that the new version of the mobile operating system that runs on iPhone, iPod Touch, iPad and iPad Mini devices represents the biggest platform shift since the original iPhone. For retailers, that’s good news and bad news.

The good news is that iOS 7, which officially launches Wednesday Sept. 18, offers retailers new features and technologies that could boost mobile commerce efforts. For example, iOS 7’s iBeacon can be used to send a consumer a coupon when she enters a retailer’s store.

The bad news? 90% of iPhone and iPad apps designed to run on Apple’s previous iOS 6 perform improperly or crash on iOS 7, mobile app testing firm uTest Inc. says.

For the past two months, uTest set 400 of its testers in nearly 100 countries to test more than 100 mobile apps, many from the retail realm. The company conducted the tests on mobile devices running the beta version of iOS 7, which was made available in June to Apple developers. The testing firm declined to reveal the names of the companies whose apps it tested, but did say that its retailer clients include Amazon.com Inc. (No. 1 in the Internet Retailer Top 500 Guide),  J. Crew Group Inc., Ralph Lauren Media LLC, GameStop Corp., Tesco Stores, Marks & Spencer and The Container Store Inc.

“Retailers must recognize that the leap to iOS 7 is greater than prior iOS version jumps from Apple and that they need to be much better prepared to make this leap successfully,” says Matt Johnston, chief marketing officer at uTest. “Retailers need to test their applications. Test them in their quality assurance labs, yes. But also test them under real-world conditions that mirror how and where their users will be experiencing the apps. There’s never been a better advertisement for the necessity of ‘in-the-wild’ testing than this migration to iOS 7.”

A majority of iOS device owners with devices eligible for the iOS 7 update (iPhone 4 and up, iPad 2 and up, iPad Mini, and iPod Touch fifth generation and up) will update within one week of launch, predicts Jeremy Black, director of retail and business development at mobile app developer Xtreme Labs Inc. He has worked on apps for the likes of 3M, Fandango and Groupon. Black agrees with Johnston that there will be issues, especially ones surrounding user interfaces, with iOS 6 apps running on iOS7.

“Apps built using the iOS 6 software development kit will run on iOS 7; however, our experience has been that when you take an existing code base, built on the iOS 6 SDK, and run it on the iOS 7 SDK, it is likely to introduce bugs that will require fixing,” Black says. “Apps are going to break once iOS 7 launches. Building on the new iOS 7 SDK will require some time spent adjusting the user interface. If Apple requires that new app updates be built on the iOS 7 SDK, retailers are going to have to factor in additional time to address any user interface bugs that are introduced as a result of moving their app build from the iOS 6 SDK to the iOS 7 SDK.”

Apple did not immediately respond to a request for comment.

Johnston says testing has found three common problems when trying to run an iOS 6 app on iOS 7. The first is user interface rendering issues. Many of these issues center on iOS 7’s Dynamic Type text-sizing feature that enables users to change the size of text in apps that support Dynamic Type. In the uTest testing, text was running off of the screen and not fitting in buttons. Some apps that use custom interfaces instead of basing their interfaces on Apple’s UIKit experienced rendering issues, Johnston says.

The second common problem is gesture confusion. The new operating system adds new gestures to the mix, including swiping up from the extreme bottom edge of the screen (which takes a user to Apple’s new Command Center) and swiping left to right from the extreme left edge of the screen (which is the equivalent of the Back button). Apps with buttons or calls to action very close to the bottom edge or left edge of the screen are creating conflicts and not performing as intended, Johnston says.

“It’s important to note that neither of these gestures should have an impact on regular vertical scrolling or swiping within an app—only when the gesture starts nearer to the screen’s edge,” he says. “But it’s an easy thing for retailers to get wrong or for users to get wrong. And the result will absolutely be user dissatisfaction and lost sales for the retailer.”

And the third and most troublesome common problem is crashing. Apple has stopped supporting (known as “deprecated” among developers) some application programming interfaces, or APIs, changed others, and introduced 1,500 new ones, Johnston says. An API is a predefined interface that a developer, such as a retailer, can integrate into its app to perform predefined tasks without the developer having to write custom code to achieve those tasks. For example, an API might draw from a weather web site the temperature in an app user’s location.

“If an API disappears or is changed, the retailer’s application that is trying to talk to that API doesn’t necessarily know how to handle the changes and causes a crash,” Johnston says. “As for deprecating, Apple has a fairly generous ramp for deprecating APIs, they won’t necessarily shut something off right away, but they will stop supporting it over time. In addition to APIs deprecated from iOS 6 to iOS 7, we’re also seeing the end of life for iOS 5 APIs that were deprecated in the transition to iOS 6. This is mainly a concern for those retailers with older, more mature apps that have been around for two or more years, but haven’t necessarily stayed current in how they interact with Apple’s evolving base of APIs.”

It’s important to note that API changes are not the only causes of crashes in the uTest testing, Johnston says. Other factors that contribute to app crashes include memory management, Internet connectivity and code conflicts.

Some retailers in mobile commerce have been preparing extensively for the iOS 7 launch. At web-only apparel retailer JackThreads.com, where 50% of total sales are mobile and the majority of mobile sales come through its iPhone app, technology staff members are confident their iOS app is ready for the change in platforms. They began preparations by attending this year’s Apple Worldwide Developers Conference in June, where iOS 7 was unveiled.

“We started testing against early betas of iOS 7 literally during lunch breaks at WWDC to identify any major concerns early on,” says Annie Trombatore, director of product development at JackThreads.com. “Luckily our apps don’t have any of the major structural bugs I’ve seen present in other apps as I started using iOS 7 beta on my personal phone. We’re lucky in that we’ve kept our apps clean and simple, so a lot of the gesture confusion being spoken to doesn’t really apply to us. The very minor, stylistic bugs are something we’ve already been able to remedy ahead of the launch, for the most part. That being said, the new features that iOS 7 brings natively are definitely something we’re super thrilled about. While we’ve ensured that our apps are stable and relatively bug-free prior to the launch, we’re excitedly gearing up to make more drastic changes to the apps to embrace these new standards and cleaner design language.&rdquo

Johnston says uTest has made requests for iOS 7 app testing by existing and new clients its top priority. He declines to reveal the cost of testing.

Apple’s App Store boasts 900,000 apps, the company reports. Apple adds there have been 50 billion app downloads since the App Store opened in 2008.

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