Here's why WP current phone will not get Windows Phone 8

Microsoft caused a lot of excitement when they announce Windows Phone 8, but also a lot of disappointed when it became clear that the new OS will not come to the current crop of Windows Phone devices.
There is a reason for it - two OS may look similar on the outside, but inside they are very different. The new core allows so many cool features (multiple core CPU, better graphics) divided by 8 RT Windows (Windows NT-based) and not Windows Phone 7 (based on Windows CE, the same as Windows Mobile).
Senior marketing manager for Microsoft Windows Phone, Greg Sullivan, said that it is not possible to port WP8 on older devices, but the cost of doing so would be very high and very little benefit - WP8 allows multi-core support, screen resolution than high, NFC, microSD card support and so on, no one will make a difference on legacy hardware.
What they get (as part of the update Windows Phone 7.8) is the new home screen - it's the most noticeable changes in the OS and will truly benefit the current Windows smartphone phone.
Users who move to a new version of the OS will get to keep using their Windows Phone 7 applications such as who will work in WP8. There is no backwards compatibility for WP8 new applications are developed though, and nothing can be done about it - ie, Adreno 205 just will not cut it for a game aimed at S4 Snapdragon (which packs a Adreno 225 or 305).