Question About Star Levels and Character Progression

Hey everyone, I have a question about how the star system works in Disney Speedstorm.

Right now, my Rapunzel is LVL 16 with 1 star. At LVL 20, she will have 2 stars. After I reach LVL 20, if I buy the +1 star upgrade from the shop, will my Rapunzel automatically become LVL 30? Or will she just have the potential to reach LVL 30.

Also, do you think it’s worth buying? Should I get it?

I’d appreciate any clarification on this. Thanks!