The secret to rebuilding your brand

"It would be best to track brand sentiment before, during, and after the rebranding." <- What if brand sentiment becomes less favorable after the re-brand? Do you go back to the old version or do you give it time? Also, the focus group you're getting feedback from would already know the old brand (i.e. they represent "existing customers" in this sense), so this approach wouldn't be great at gauging the preferences of new customers.