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 were “standardised” (changed to their -o forms, or eliminated entirely) in the Koine period and the transformation was nearly complete by Byzantine times, with remnants of the -mi conjugation persisting only in the Passive voice in Modern Greek.