Khi nào dùng `change` và khi nào cần viết `up`/`down` riêng trong migration?

def change
  add_column :users, :age, :integer   # down: remove_column
  add_index :users, :email             # down: remove_index
  create_table :posts do |t| ... end  # down: drop_table
end

def up
  execute "UPDATE users SET role = 'member' WHERE role IS NULL"
end

def down
  raise ActiveRecord::IrreversibleMigration,
    "Cannot reverse a data-only migration"
end