When to fix something that is "wrong" but works fine

Short version:

I wrote some code that is not done "the right way" because, when I wrote it, I didn't know how to do it. Now that I know how to do it "the right way", how do I decide how to fix it?

Longer version with details:

java.util.DecimalFormat is not Thread safe. I have what's essentially an immutable Description object that is used all over my code over many different threads. To fix this problem, I created a wrapper class SynchronizedFormatter that basically just wraps a lock around the methods of DecimalFormat that I use.

The right way to do this is to use a ThreadLocal . I didn't even know what that was when I wrote the code the first time. Is it worth it to fix? The synchronized version works just fine. How do I decide, about this in specific and about this situation more generally?