Also, the transition to Windows Phone might take too long to allow Nokia to compete, given the ongoing development of other platforms, Nokia said.

In the same filing, Nokia said it expects to take two years to make the transition to Windows Phone as its primary platform. During that time, it will continue to support Symbian in the hopes of transitioning the installed base of 200 million Symbian owners to Nokia Windows phones, Nokia said. It reiterated its hopes of selling an additional 150 million Symbian phones in future years.

Nokia pointed to other threats as well, such as the danger that the deal could erode its brand identity in areas such as China, where it is quite strong, and fail to enhance the brand in areas such as the U.S., where it is currently weak.

Another challenge will be building a profitable business model around a platform like Windows Phone, for which Nokia must pay royalties, Nokia said. While Symbian is a royalty-free operating system, it didn't come without significant costs to Nokia, which spent more than US$400 million in 2008 to buy the half of the Symbian company that it didn't already own.

In the filing, Nokia also warned about the challenges it faces internally as it implements the new plan for Windows Mobile. For example, it may not be able to change its mode of working or culture to work effectively with Microsoft, the company said. Because Nokia anticipates laying off a large number of workers, the remaining employees may lose motivation, energy and focus, thus reducing their productivity, Nokia said.