In PerfectIt I noticed there are two versions of my style, one with (Old) next to it. What is the difference between these two versions?

When we release a major PerfectIt for Word (Windows) upgrade (e.g., from version 4 to version 5), PerfectIt makes sure you keep your old styles.

PerfectIt imports them into the updated version and adds the ending "(old)" so you have both your previous version as well as a new one that contains any updates we've added.