To extend the validity period of your remaining page credits, you can buy a new pack.
Your remaining page credits will be added to the newly purchased pack and the maximum validity period will prevail.
Here is how it works. Suppose on January 1, 2015 you have 5 page credits left, valid through January 5, 2015.
If you buy the "200 pages for month" pack, you will get a total of 205 page credits valid through February 1, 2015.
If you have not found answers to your questions here, please take a look at the FAQ section.