Current Windows Phone’s are not going to upgrade to Windows Phone 8, Instead they will be upgraded to Windows Phone 7.8. Most of the software specific features are available in Windows Phone 7.8, only hardware related features are not available. It make’s sense, Without the hardware its meaning less to bring those features. Existing hardware in Windows Phone platform doesn’t have Dual-core or NFC, getting support for these doesn’t mean a lot. But you are going get all software related features, For example the new Wallet in Windows Phone 8 uses NFC for “Tap-to-Pay”, it uses the NFC hardware, the same wallet app will be used for in-app purchase in the phone, and you are getting that in Windows Phone 7.8 for your current device, its make sense.
Microsoft didn’t shared much about the consumer features in Windows Phone 8, the event is focused on platform and developers. What we seen so far is the new start screen UI, and you are getting this for current devices through Windows Phone 7.8. And you are not stopping there, you will get lot of end user features which we don’t know yet. With the Windows Phone 7.8 update you will get the look and feel same as a Windows Phone 8 device, only thing is you wont have a new hardware.
And all the present Apps will work in Windows Phone 8, But the apps which are developed exclusively for Windows Phone 8 will not available for present phones, because of the lack of hardware support.