title: Stripe: change plan, don't prorate unused time
body: <p><strong>Scenario:</strong></p>
  <p>On the 1st of May, the user begins his subscription at 25€/month.
  On the 15th of May, he switches to an upgraded subscription at 50€/month. He is immediately invoiced 12.5€ (25€ difference - 12.5€ unused time) I believe.</p>
  <p><strong>I would like the user to pay the full 25€ difference while keeping the billing period the same.</strong></p>
  <p>How could I remove the &quot;Unused time&quot; from the equation? Is there a simple way?</p>
  <p><em>Additionally, if the user downgrades, it would be optimal if the user receives no credit to his account.</em></p>