Then either the contract has changed or you got a one time good deal. When I tried to do it in 2014, it was reversed. Here is a quote from the current SRH:

Q: What happens to my payback days if they are not used in the current year?
A: If the days have not been used prior to January 1, they will automatically be converted to supplemental (SUPP) vacation days to be used in the subsequent vacation year.If you wish to use the days in January, February, or March, you may enter a request for a personal drop (PD), or APD and when awarded via PCS, contact a Crew Scheduling supervisor to have the days of the personal drop converted to payback days, if desired . This will be shown on your Schedule as TOFF .

Denny