Mi-verbs were “regularised” (transformed to their -o counterparts, or replaced altogether) in the Koine era and the transformation was almost complete by Byzantine times, with some vestiges of the -mi conjugation surviving only in the Passive voice in Modern Greek.
Mi-verbs underwent standardization (became -o verbs or disappeared) during the Koine period and became nearly uniform by Byzantine times, with minor remnants of the -mi conjugation persisting only in the Passive voice in Modern Greek.