I've never had a schedule change or cancelation with Viva Aerobus but I have had several with Volaris. They are usually very accommodating and offer to either refund the ticket, give an electronic credit for an amount over and above the ticket's value, or change the flight to another flight of my choosing at no additional charge. I would keep asking Viva and see if someone budges on your request. That said, this is a pretty minor schedule change (even by relatively generous US standards) so that may explain why they are being more difficult.