Preserving behavior is a large challenge. When we need to make changes and preserve behavior, it can involve considerable risk.
To mitigate risk, we have to ask three questions:
What changes do we have to make?
How will we know that we've done them correctly?
How will we know that we haven't broken anything?
How much change can you afford if changes are risky?
Most teams that I've worked with have tried to manage risk in a very conservative way. They minimize the number of changes that they make to the code base. Sometimes this is a team policy: "If it's not broke, don't fix it." At other times, it isn't anything that anyone articulates. The developers are just very cautious when they make changes. "What? Create another method for that? No, I'll just put the lines of code right here in the method, where I can see them and the rest of the code. It involves less editing, and it's safer."
It's tempting to think that we can minimize software problems by avoiding them, but, unfortunately, it always catches up with us. When we avoid creating new classes and methods, the existing ones grow larger and harder to understand. When you make changes in any large system, you can expect to take a little time to get familiar with the area you are working with. The difference between good systems and bad ones is that, in the good ones, you feel pretty calm after you've done that learning, and you are confident in the change you are about to make. In poorly structured code, the move from figuring things out to making changes feels like jumping off a cliff to avoid a tiger. You hesitate and hesitate. "Am I ready to do it? Well, I guess I have to."
Avoiding change has other bad consequences. When people don't make changes often they get rusty at it. Breaking down a big class into pieces can be pretty involved work unless you do it a couple of times a week. When you do, it becomes routine. You get better at figuring out what can break and what can't, and it is much easier to do.
The last consequence of avoiding change is fear. Unfortunately, many teams live with incredible fear of change and it gets worse every day. Often they aren't aware of how much fear they have until they learn better techniques and the fear starts to fade away.
We've talked about how avoiding change is a bad thing, but what is our alternative? One alternative is to just try harder. Maybe we can hire more people so that there is enough time for everyone to sit and analyze, to scrutinize all of the code and make changes the "right" way. Surely more time and scrutiny will make change safer. Or will it? After all of that scrutiny, will anyone know that they've gotten it right?