You will have more wear on your new inner tires with worn out duals. We had that experience with a 4430 with Firestone inner tires and Goodyear duals the goodyears were a little smaller diameter and the firestones did not last nearly as long as the other 4430 with all Firestones these were all 18.4x38 tires . I just put all new ones on when replacing drive tires. You will not hurt anything just have more wear on your new tires.