Assassin's Guild also became Assassins, in BM.
As for name changed in real medieval times, it did happen.. Like when the Kingdom of the Franks divided, and West Francia became know as the Kingdom of France during the Capetian Dynasty. Or, for the best example would be the Holy Roman Empire, in either incarnation. In fact, the HRE named such because of the lands it possessed and the declaration that they we're a continuation of the Western Roman Empire.
Given some stability over time, some penalties for name changes, since it's not like he's conquered so much more land and forming a new empire, I don't see it as too far fetched to see his kingdom name being changed to reference his past kingdom. Heck, Otto I took the name of Charlemagne's HRE when he felt like it, just like Charlemagne more or less did when he declared himself such after the western empire.
Let him have have his old cake, but give him some belly ache to let him and others know that it doesn't digest well.