At New London, "Theodore Roosevelt" entered the shipyard of the Electric Boat Division for an extensive overhaul. Between July 1965 and January 1967, her nuclear reactor was refueled and her Polaris weapon system was modified to accept the more advanced Polaris A3 missile. She completed overhaul on 14 January 1967 and began sea trials and refresher training, all of which culminated in the successful firing of a Polaris A3 missile at the Cape Canaveral missile range late in April 1967. At the end of the training period, she returned to Charleston to load missiles and to prepare for another series of deterrent patrols out of Holy Loch. She embarked upon her 18th patrol on 1 June 1967 and completed that cruise at the Holy Loch base.