If you are in a Circle of Protection, and have the ability to cast another, and get hit with a Dispel Circle or Dispel Magic…
…in order to recast it, you must retrace the circle and state “Circle of Protection.” You certainly will be vulnerable for a few seconds, depending on how fast you can do those things reasonably and clearly.
However, if, when you first cast a circle, you actually drew a circle in the dirt (or drew in chalk, or laid down a circle cord, or something else clearly visible), and it’s still clearly a circle…
…then you don’t need to retrace. You can just restate “Circle of Protection” and be in another. (You’ll still be briefly vulnerable, based on your reaction time and how quickly you can clearly say it.)
…
This will get clarified in the rulebook over the winter.