Microsoft has promised to bring Windows Phone 7.8 to existing devices and with that will come a new start screen. In the video below, your host, BenThePCguy, takes a closer look at resizing live tiles to either small, medium, or large. It is a brief look but it does show how customizing your start screen will be an easy, yet familiar experience.
While we do not have a launch date yet for Windows Phone 7.8, Windows Phone 8 will be here in time for the holiday season. Knowing this, we would expect Windows Phone 7.8 to come out around the same time as Windows Phone 8.
Knowing that existing phones will get the new start screen should bring some comfort to those with existing devices. Is it a perfect solution? No, but is it necessary and better for the longevity of the ecosystem, certainly.
Source: Windows Phone Blog