In the case of B and C races, we wouldn’t recommend deleting and re-processing your Plan Builder since the logic is fairly easy and you can add these events manually with relative ease.
For C races, simply drop the planned workout on the date of your C race and swap it for your event.
For B races, you will follow the same procedure but you can also choose to drop the ride the day before (if there is one) and insert a race opener workout.
In cases where your A race has changed, however, deleting and reprocessing your plan will be the best option since it will tailor your training to help you peak for your A priority events.