When the update launches on April 9, all Racers will automatically be upgraded to the next Star.
For example, if you racer is at Level 32 Racer (equal to 3 Stars and 1 Star Fragment in the new system), they will be upgraded to 4 Stars (equal to Level 40 in the current system) when the update launches.
Note that Racers at Level 10, 20, 30, 40, and 50 won't be upgraded to the next Stars, and they will remain at Star 1, 2, 3, 4, and 5, respectively.
You can check what Star your Racer will get in this article here.