Let me give you an example. I passed 1.2 and 1.4 already. I have a choice of taking either the non-upgrade, or the upgrade exam. The upgrade exam contains only those that are new to 1.5, as compared to 1.2 and 1.4. The upgrade is nice for my case because it only involves the 'new stuff'.
For someone who has not passed any previous
scjp, you have no choice but to take the non-upgrade.
You need to take either one of them only, not both.