There had been rumblings that existing Windows Phone devices would not receive the update to Windows Phone 8 and that has been confirmed today. If you have a Windows Phone running 7.5, you will receive an update to Windows Phone 7.8. Some of the features in Windows Phone 8 will be included in the 7.8 update. According to The Verge, existing devices will receive the new Start Screen experience as part of the 7.8 update. Some of the features, such as support for dual-core processors, didn’t make sense to bring to older phones.
“When you pull that Lumia out of your pocket after you’ve received that 7.8 update, it will look and feel the same as a Windows Phone 8 device,” claims Sullivan. “Because you don’t have a multicore chip and don’t have some of these other elements, it didn’t make sense for us to make those investments for devices that couldn’t really exploit them.”
Source: The Verge