Dan. [ǣ] became [ā] in East Danian; [ǣ] > [ā]
According to the Comparative Tables, long [ǣ] developed into [ā] in the East Danian dialect instead of becoming the diphthong [ea] as it did in Ossiriandic (PE19/25). In rough notes associated the Comparative Tables, Tolkien ascribed this change to “West Danian” (PE19/26), but according Christopher Gilson, these changes were reassigned to East Danian as Tolkien compiled the tables themselves (PE19/26 note #85).
References ✧ PE19/26
Phonetic Rule Elements