For existing Windows Phone 7.5 users, bad news for you. Your Windows Phone 7.5 will NOT get updated to Windows Phone 8. However, as a condolence, they will provide you Windows Phone 7.8 with some limited features of the Windows Phone 8.
And here is a recap of what Windows Phone 7.5 users will get.
As you can see, some features from Windows Phone 8 will be available. The rest, I guess you will need to buy a new Windows Phone 8. BTW, Microsoft also announces that Marketplace has already reached 100,000 apps. But can you guys remove those “crap” apps?
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(source)