When the update launches on April 9, all Racers will automatically be upgraded to the next Star.
For example, if your Racer is at Level 32 (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 is launched.
Note that Racers at Levels 10, 20, 30, 40, and 50 won't be upgraded to the next Star, and they will remain at Star 1, 2, 3, 4, and 5, respectively.
You can check which Star your Racer will get in this article here.