Not possible, sorry! We already stated that once you go to that new dashboard, you will not be able to sign on or change from that until November, when the official dash update is released. Unless KingBurnz proves otherwise, you are outta luck, son.
You can only upgrade, not downgrade. If your wanting live there is nothing much you can do except wait.
There is something that you can do >.> burn the mainboard down und call microsoft & hope for a new xbox.
Hopefully you read the notices about joining the Kinect dash beta program, if you did you should be able to get back on live soon