It would be risky to rebook the earlier flight, since Croatia may not want to pay this, and it's not a water-tight aspect of EC261. It may be that via Amex you feel you can take the risk. There is a requirement to rebook on the next available service but you do need to give the airline some bandwidth to do this for you. According to their website they start at 09:00 CET, which is 08:00 BST. However they are mainstream Amadeus users, there isn't anything suggested online is there?