If the aircraft did not have a valid CofA, it can still be flown under a Permit to Fly. EASA Part 21.A.701 details the type of flying which can be carried out on a Permit to Fly, which includes "flying the aircraft for customer acceptance", "delivering or exporting the aircraft", "market survey, including customer’s crew training" and "exhibition and air show". If the aircraft had a valid CofA, it is just a normal flight. I understand that Italian register allows registrations to be re-used; for example I-RAIU has been associated with several serial numbers.