Today in the Windows Phone Summit, Microsoft clearly mentioned that, there will not be any kind of upgrades of Windows Phone 8 in Windows Phone 7.x devices. Instead of that, they will throw a upgrade to WP7.8 for all the WP7.x handsets to fulfill the transitions between Windows Phone 7 and Windows Phone 8.
This post will cover the new interesting stuff (from the Windows Phone Summit) that Windows Phone 7.8 will have to give you a look & feel of Windows Phone 8.