Impact on holidays for Casual to Flexible employment

Hello.   We have a casual member of staff who is moving onto a flexible employment status in January. He has accrued holiday whilst he has been working as a casual.   I understand that when he moves to flexible, the system will start calculating holiday based on the last 12 to 26 weeks worked on the rota. But as our holiday year starts in January and finishes in December, won't his holiday be wiped from the record?   Would it be best to pay out the holiday this month or add it to next year's entitlement?   Thanks.   Danni