The Middle Byzantine notation (MBn) is used to capture the plainchant melodies of eastern Orthodox Christian music from the middle of the 12th century until 1814. In the context of this research, we study the evolution of a subgenre of Byzantine music known as Heirmologic. We present three Heirmologic corpora spanning the periods before, during and after the 16th century. We discuss the challenges we faced during the digitisation process, and the steps we took to overcome them. For the analysis of the three corpora, we apply the three methods, namely notational texture, melodic arch similarity, and Jensen-Shannon distances of Markovian models, the second of which is novel and inspired by the idea of melodic arches. Through these methods, we aim at highlighting the differences of the corpora in order to obtain an outline of the evolution of the subgenre. We observe that the post 16th century Heirmologic pieces are more similar to the 16th century ones, while there is a greater difference with the pre 16th century pieces. This indicates that the 16th century constitutes a turning point in the melodic features of the Heirmologic subgenre.