Use Request event change in the portal (or from your booking details). Choose the event you are booked on and the event you would like to move to.
Subject to availability, a change is free up to 28 days before the event. Inside 28 days there is a £50 plus VAT admin fee. Change requests are hidden in the last 7 days before an event.
Your requests, their status (pending, approved, declined) and the full audit trail appear in the Event change requests section of the portal, and you are emailed when a decision is made.