Embracing change is a core agile value. That does not mean that we should always say "yes". As developers, we acquire detailed knowledge of the domain and the system. With this knowledge comes the responsiblity to say "no" when we see a real problem. By saying "no", we make room for discussions about the issue at hand, and make it possible do make informed decisions. If we don't say "no", we make promises we can't keep. That means more risk, more stress, lower quality.