Dragonlords, book 1: The Last Dragonlord
Joanne Bertin1998

For aeons, the Dragonlords (humans who can become dragons at will through ancient magic) have helped true humans, sitting in judgement on their wars and political wranglings. Linden Rathan, youngest of the weredragons, is sent to Casna in the land of Cassori, there he must judge a regency dispute.