According to the prayers of the services to St. Michael, he is both Seraph and Cherub.
I have copied some of the comments below from:
https://www.facebook.com/groups/ask.abo ... 266995144/
"Fiery your form and wonderful your beauty, Michael first of Angels, for with your immaterial nature you pass through the ends of the world fulfilling the orders of the Maker of all things, recognised as powerful in your strength, and making your temple, honoured with your holy name, a source of healings." (Vespers, Tone 4)
Here's a line for both Michael and Gabriel: "Bright with the sun’s beam of the divine light, Chief Captains, you outshine the ranks of immaterial beings; clothed like light in dazzling white in the highest, you shed on the world the fire of the unapproachable Godhead; and so you sing unceasingly with lips of flame the thrice-holy hymn: Holy, Holy, Holy are you, our God. Glory to you."
from Matins, Ode 6 Canon 2: "Put to death, O Archangels, the assaults of the passions, and with them bring to an end all the scandals of the faith, O Holy ones formed of fire, the Chief Captains of the Trinity."
The comments below were written by a priest, Archpriest Sergey Muratov:
Here you see archangels are of lower rank. Saint Michael is called archangel in broader sense.
https://books.google.ru/books?id=GXkB8jVstroC&pg=PA43...
Heavenly hierarchy works this way. It is not democratic possibility for lowest to become highest. Michael was one of 8 highest Angelic spirits, with Gabriel, Raphael, and Lucifer. Lucifer did rebellion against God and Michael fight him down.