Front tires on a bicycle generally don't wear enough to make any difference to safety. If the tire is showing other signs of age, like lots of small cuts, or dried out and cracked casing, then it is probably a better idea to put it on the rear and put the new tire on the front, but if there is no noticeable damage or wear beyond missing the 'hairs' that are formed on new tires from the manufacturing process, then there is no advantage to replacing it.