Most people by now would have known our capital, Pekan Brunei was renamed Bandar Seri Begawan in honour of Al-Marhum Sultan Haji Omar Ali Saiffuddien Sa'adul Khairi Waddien the Third, the 28th Sultan. If not, then you can read it at this post here. The name Seri Begawan Sultan was used by His Majesty when he abdicated or stepped down from the throne in 1967 thus installing His Majesty Sultan Haji Hassanal Bolkiah Muizzaddin Wadaullah as the 29th Sultan.
Begawan is the title used to denote someone who has 'retired' for Bruneians though it has a slightly different meaning in standard Malay. Thus if you are titled Pehin Khatib and you have reached retirement age your title changed to Begawan Pehin Khatib. Thus in Sultan Haji Omar Ali's case, he becomes Begawan Sultan.
In the 600 years of Brunei's Sultanate, the title Begawan Sultan has been used six times. Other than Sultan Haji Omar Ali Saifuddien III, the other five Sultans who voluntarily abdicated are:
Sultan Sulaiman, the 4th Sultan reigned from 1432 to 1485 and step down to allow his son, Sultan Bolkiah to become the fifth Sultan. Sultan Sulaiman passed away in 1511;
Sultan Abdul Kahar, the 6th Sultan reigned from 1524 to 1533 and step down to allow his son, Sultan Saiful Rijal to become the seventh Sultan. Sultan Abdul Kahar passed away in 1538;
Sultan Shah Brunei, the 8th Sultan reigned from 1581 to 1582 and step down to allow his younger brother, Sultan Mohammad Hassan to become the ninth Sultan. Sultan Shah Brunei was very old when he became the Sultan;
Sultan Husin Kamaluddin, the 16th Sultan is the only Sultan to have step down twice and became Begawan Sultan twice. He first reigned in 1710 when Sultan Nasruddin passed away and he step down in 1730 to allow his nephew, Sultan Muhammad Alauddin to become the 17th Sultan. But Sultan Muhammad Alauddin passed away in 1737 and his son was too young to become the Sultan. Sultan Husin Kamaluddin became the Sultan again until 1740 when he step down for the second time to allow the young prince, now grown up to become Sultan Omar Ali Saifuddien the First, the 18th Sultan;
Sultan Muhammad Tajuddin, the 19th Sultan reigned from 1795 to 1804. He step down to allow Sultan Muhammad Jamalul Alam to become the 20th Sultan. Sultan Muhammad Jamalul Alam died within 8 months of becoming Sultan and Sultan Muhammad Tajuddin took over as the Acting Sultan while waiting for the rightful Prince to grow up. Unfortunately it took a while for the rightful Prince to grow up and by then the throne was usurped by Sultan Muhammad Alam who ruled until he died before the throne was rightfully given back to Sultan Omar Ali Saiffudien II.