After this update, all karts, characters and tracks you unlocked are removed. You start over, but all the karts that you purchased before are converted into gems, so it’s not totally starting from zero.
From the Google Play store:
We’ve listened to your feedback, and after a quick pit stop, we’re happy to introduce Angry Birds GO 2.0!
– IMPROVED CAMPAIGN MODE! The campaign gets a total overhaul.
– NEW UPGRADE SYSTEM! Collect parts and upgrade your karts.
– LIMITED-TIME TOURNAMENTS! Compete to win prizes!
– DAILY RACES! Enter a new race every day and find a hidden gift box.
– NEW HANDLING SYSTEM! We’ve tuned every kart for better handling.