Buying a bicycle isn't really a one-step process. It can be, but it rarely is. Because what was the "perfect bike" at the shop doesn't stay that way for long. And whether it's reconsideration via a long ride or just trying out someone else's bike, the urge to upgrade is inevitable. Sometimes "just because" is reason enough.
The first step should be simply accepting that it's going to happen. The prudent thing to do is to replace parts as they wear out, but what fun is that? So think before you buy, and, of course, read this. Here are 10 Ways to Upgrade Your Fixie.