Your tickets will hold value to be used towards a future visit. You can either make that change now (or between now and your cancelled April arrival) if you know the new dates, or do it later. If you do it ahead of the April arrival, you should be able to make that change yourself online. If you wait until after the tickets expire, you will need to call; be aware the tickets will disappear from your view after expiration but WDW can still see them. I recommend making note of the ticket numbers and value (WDW online value for those dates at the time you purchased the tickets).