Some companies have had different periods for earning holidays (say Jan to Dec) and taking holidays (say April to March). But I thought companies were moving away from this because of the new rules regarding holiday entitlements. If they make you take 12 months holidays over 15 months then this would only be okay if your pro-rated holiday entitlement over the next 12 months was more than the statutory minimum entitlement.
If the change was within the rules then you would eventually get the outstanding 3 months holiday entitlemnt when you leave.