Your auto policy will generaly provide you coverage for a new veh with your current card up to 30 days. Depending on carrier, the amount and length of cov will vary, but at the min you will have liability cov.
From what I've learned in this discussion, while a car me be new to you, unless purchased new from a dealer, it may be considered used in the eyes of the insurance company, and interpreted that way in court. Also, you are risking, at officer discretion, and impound, hefty fines and a large headache.