The main reason the new software won’t run on older phones is that most of the new changes are related to support for new kinds of hardware that don’t exist in the current generation of phones.
By contrast, apps written for Windows Phone 7 will run on both current and future phones. That’s because even though Microsoft is changing the core of the operating system, it has never allowed programmers to write natively for Windows Phone 7.
However, apps written for Windows Phone 8 won’t necessarily run on the older devices, meaning that those with older devices may soon see cool apps that they can’t use.