I downloaded Xcode beta and opened my existing project to see how it works with Xcode 8 beta (I opened Storyboard file from Xcode 8).
Now when I open my project from Xcode 7.3 I am not able to open storyboard file , it gives me following error
This version does not support documents saved in the Xcode 8 format.
Open this document with Xcode 8.0 or later.
I have figured it out. I opened Storyboard in Xcode 8
Open Interface Builder Document and select opens in Xcode 7.x instead of Latest Xcode (8.0) as in following screenshot
On doing this, you will get an alert saying "Saving for Xcode 7.x will close your document and data for Xcode 8.0 features will be removed."
Click Save and Close and you will be able to open storyboard in Xode 7.x.