Source -It's long been known that Microsoft would have much tighter hardware requirements for Windows Mobile 7 than any previous version of its mobile operating system. Therefore, the general expectation for upgrades to the new version have not been hopeful: if you want Windows Phone 7 Series device, the fact that you have a Windows Mobile phone right now does not mean you can avoid buying a new phone.
WinMob 6.5 to Windows Phone 7 Series upgrades to be rare