"Tiru" returned to Pearl Harbor for a major overhaul, entering the shipyard on 4 May 1959 for a fleet rehabilitation and modernization (FRAM) conversion to a "Guppy III". In the course of the work, the ship took on a new and different external appearance. Her hull was lengthened by ; she acquired a new conning tower, longer than its predecessor; and a fiberglass sail was added. Internally, increased sonar and ordnance equipment greatly enhanced the ship's capabilities in those key areas. On the last day of the year 1959, "Tiru" emerged from the overhaul a virtually "new" ship.