Starting a Major Update to a Shaky Application

Today I started adding several new fields to an application that is targeted for a re-write, but we still need to get many more miles out of it before that point in time. The problem is, it's very brittle in the sense that I'm still not sure what touches what within the code. Sure, I'm getting closer to having a good understanding, but this level of change makes me more than a little nervous.

Still... if I take it slow, and make sure I make all the changes in the interfaces, and the factories, and the classes and the output, I should be able to get this done in one pass. That's my goal anyway.