ALTER TABLE subscription_payments
  ADD COLUMN IF NOT EXISTS title text,
  ADD COLUMN IF NOT EXISTS amount double precision;

UPDATE subscription_payments AS payment
SET
  title = subscription.title,
  amount = subscription.amount
FROM subscriptions AS subscription
WHERE
  subscription.id = payment.subscription_id
  AND (payment.title IS NULL OR payment.amount IS NULL);

ALTER TABLE subscription_payments
  ALTER COLUMN title SET NOT NULL,
  ALTER COLUMN amount SET NOT NULL;

ALTER TABLE subscription_payments
  DROP CONSTRAINT IF EXISTS subscription_payments_amount_positive;

ALTER TABLE subscription_payments
  ADD CONSTRAINT subscription_payments_amount_positive CHECK (amount > 0);