Do I have to pay full price if I want to add more names later?

No, you only pay the difference in price to add more names, plus any upgrade charges for a new version.

For example, if you purchase a 250 name unlock code and later decide you want a 500 name unlock code, you'll only pay the difference in price between the 250 and 500 name unlock codes. (The price difference is calculated at the time you purchase the second code, not the first. In other words, if the price of the 500 name code has increased since you purchased your 250 name code, then the difference in price may have increased as well).

When you decide to add more names at a later date, you may also be required to purchase an upgrade to the latest version of ChurchTrac at the same time (getting the latest update for ChurchTrac 2008 is free; going from ChurchTrac 2008 to 2009 requires a small upgrade fee). At some point after a new version of ChurchTrac is released, unlock codes for the older version will no longer be available for purchase. The price of the upgrade will be added to the cost of increasing the number of names you can enter. Upgrades allow you to get the new features and stability of the newer version at a fraction of the cost of purchasing at the non-upgrade rate.