The edict of Emperor Shomu as well as that of Empress Genmei can be easily understood, if you suppose that Fukai-no-Joten/ Fukaijoten was brought forward in order to cope with the old practice of the brother to brother succession.
兄弟継承の古い慣習に対抗するために不改常典が持ち出されたと考えると、聖武天皇の詔も元明天皇の詔も理解しやすい。