This name change, just like many among Japanese nobility, occurred with a Fujiwara member.
Fujiwara no Kanezane (1149-1191)
He was a descendant of the Fujiwara clan.
After becoming regent and then Chief Minister, he was encouraged to found his own clan by Minamoto no Yoritomo.
Kujō Kanezane (1191-1207)
Kujo (九条): nineth article
The name Kujo derived from a house built by his Fujiwara ancestor that overlooked a road named Kujo-Oji where Kanezane and his family lived.
Sources
- Saeki, Umetomo; Mabuchi, Kazuo (1969). 講談社古語辞典. 講談社. p. 300.
- Sansom, George (1958). A History of Japan to 1334. Stanford University Press. p. 322-324.
- Wikipedia.
- \”Fujiwara no Kanezane.\”
- \”Kujō family.\”