I would disagree with swajames. Prepayments typically appear as a credit to your bill / folio on check out. If you stay after the VAT change but pay before, then you will have a credit as the charge for the stay occurs on the day of the stay. I would think of the payment as a deposit. I would be pretty sure you would get the credit back or it would offset any other charges you may have.

This assumes you booked thru Hilton and not a third party booking agent.