Moving columns to a new empty table, the foreign keys are already in place.

move_attributes_to_registrant_model.rb

class MoveAttributesToRegistrantModel < ActiveRecord::Migration
  def up
    Payment.find_each do |payment|
      registrant = Registrant.create! email: payment.email,
                                      first_name: payment.first_name,
                                      last_name: payment.last_name
      payment.registrant_id = registrant.id
      payment.save!
    end

    remove_column :payments, :email
    remove_column :payments, :first_name
    remove_column :payments, :last_name
  end

  def down
    add_column :payments, :email, :string
    add_column :payments, :first_name, :string
    add_column :payments, :last_name, :string

    Registrant.find_each do |registrant|
      payment = Payment.find_by! registrant_id: registrant.id
      payment.update! email: registrant.email,
                      first_name: registrant.first_name,
                      last_name: registrant.last_name
      registrant.delete
    end
  end
end

EDITS MIGRATION file that was not yet in production.
Removes selecting where email was not missing, which irreversibly deleted records outside this case.
Also uses bang methods so any failed validations will cause the migration to abort.