<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Here-I-Go:
Just got off the phone with Hilton and was told the certificates can be redeposited, but cannot be applied to a new reservation (they would be honored if they were tied to an existing reservation made before the point system change.</font>
You were given an incorrect information.
Adam Burke posted on FT the official answer - as long as you have a cert in hand, it will be valid for either existing, or a new reservation for a full year since the date of issue, provided that the property you want to stay at was eligible for the type of cert you want to use prior to the change.