"K'Sheves ha'Melech" implies that only now he reigned. Verse 3 says "bi'Shnas Shalosh l'Malcho"!
Megilah 11a: "K'Shevet" means after his mind was Meyushav (settled). He said, Belshatzar miscalculated the 70 years of Galus Bavel. I calculated correctly. (They finished now, in his third year. Since Yisrael were not redeemed, they will not be redeemed; he used Klei ha'Mikdash in his party.)
Rashi: "K'Shevet" means when his kingship solidified in his hand.
Malbim: Immediately at the beginning of his kingship, he was bold to move the capitol from Bavel to Shushan. He did not fear lest they rebel against him for this.
What is "Kisei Malchuso"?
Vilna Gaon and Torah Temimah citing Esther Rabah: He wanted to sit on Shlomo's throne, but they did not let him 1 . Therefore, he made his own throne like Shlomo's.
Torah Temimah: It does not say who did not let him. Perhaps it was his nation, when they made him king. Agados say that the Chayos on Shlomo's throne did not let him.
What is the significance of "Asher b'Shushan ha'Birah"?
Vilna Gaon: The capitol was Eilam; Shushan was the stronghold in it. Mordechai was already there (2:5); Hashem arranged that the capitol be moved to there, to prepare the salvation.
Malbim: Normally, if a commoner became king, he is honored to sit on the throne of through previous kings. He was so conceited that immediately he made a new throne and changed the capitol, to show that he rules via his power, and not due to their desire.
Malbim (1): A limited king cannot relocate the capitol. Initially, the 127 Medinos were of Malchus Bavel, and the capitol was there - "Heichal Malchusa Di Vavel" (Daniel 4:26).
