Place sweet potatoes into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, 40 to 50 minutes. Drain water; peel and mash sweet potatoes in a large bowl until smooth.

Preheat the oven to 425 degrees F (220 degrees C).

Mix together beaten eggs and melted butter in a medium bowl until well combined; stir into mashed sweet potatoes. Mix in milk, sugar, flour, vanilla, nutmeg, baking powder, and lemon juice until smooth; pour mixture evenly into pie shells.

Bake pies in the preheated oven for 10 minutes. Decrease oven temperature to 350 degrees F (175 degrees C) and continue baking until a toothpick inserted into the center of a pie comes out clean, about 40 minutes more. Cool pies for 15 to 30 minutes before refrigerating to cool completely, 1 to 2 hours.