May I ask why your only allowing 10% to view? Is this a way of A/B testing the new design?
Regardless, you'll want to keep your new designed pages Noindexed, nofollow until you are ready to push them live as a whole.
How many of the top level url's are you changing? From my experience, 301'ing a large amount of pages will result in a loss of traffic. If you can manage and spread them out over a time period you may have better luck.