If your contract has the original number of days then this is still what you are entitled to. It doesn't sound like you have signed anything yet to agree to the reduction, is that correct? If so, as Purple says, the best thing is to have a conversation again as nothing can be changed without you signing to agree to the changes. If the girl who left had the same amount of holidays in your contract then personally I would not be signing anything to agree to reduce these so be prepared to be firm with the boss on this.