With other parts you can just upgrade a part and select which chassis you want it to be for. But for the chassis you have to build a new chassis for the next season.
So upgrading is just like doing any other upgrade on a current part but would only apply to this season
Building new based on you would use to build a new chassis for next season based on this years chassis. So essentially the same as upgrading but having to build a new one.
Don't bother using build new based on for this season imo
Upgrade chassis: is continuously upgrading the chassis based on its basic potential and is completed quickly.
New Chassis (based on): Used to move chassis from this season into the next season as every season you need a new chassis but it can be based on the previous chassis so that you don't lose development from season to season.