It depends.

If you have previously used a promo code while subscribing to a plan, you won't be able to apply a new promo code when changing your subscription plan.

On the other hand, if you haven't used a promo code before and have cancelled your subscription from Settings or if it was automatically cancelled after the grace period due to a failed credit card payment, etc., you will be able to apply a promo code when you renew your subscription from the login page.