Keep the option for the older interface around
I feel that your new version suffers from "redesign fever", where you've attempted to make it so pretty that you remove the utilitarian functionality I like in the current interface.
Just like Gmail when it went through its redesign, your new version has much less information density, and I feel that you're losing visual distinction between sections of the interface.
Please allow us to keep the current interface around.
We’ve disabled the ability to see both UI’s due to the amount of engineering time it would take to maintain both sides. If you have any specific comments on what you’d like to see improved in the new UI please feel free to add Uservoice tickets.