This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
A voucher has a validity date, which cannot be changed.
If you are referring to the date you scheduled the exam, then you'll have to ask the center where you scheduled the exam. At the prometric center where I scheduled the exam, it was written that we could change the scheduled date for free, as long as it was done 3 business days before the scheduled date. After that, it cannot be changed.