Microsoft had originally planned to continue the Windows Mobile line to Windows Mobile 7, based on an upgrade to the Windows Mobile platform, codenamed Photon. The original Photon and Windows Mobile 7 have since been scrapped, however, Microsoft decided to create a new mobile OS platform and officially announced Windows Phone 7 Series in its place. Microsoft has since renamed the operating system from Windows Phone 7 Series to Windows Phone 7.
Windows Phone 7 was initially intended to be released during 2009, but several delays, likely due to the move away from Photon and to building an entirely different and new platform, prompted Microsoft to develop Windows Mobile 6.5 as an interim release. During the Mobile World Congress 2010 in Barcelona, Microsoft revealed details of Windows Phone 7, which features a new operating system and integration with Xbox Live and Zune services.
Phones currently running Windows Mobile 6.x will not be upgradeable to Windows Phone 7.