At some point in the project, someone such as an end user will ask if something could be changed or a new requirement be supported. We’re trying to maintain scope, so the answer is usually NO.
You wouldn’t like it so why should the requestor. Look for ways that you can say Yes. It might be more of a qualified yes.
Can you add feature G? “Yes, in the next quarterly release.” Notice that you didn’t say NO. You played with the when factor.
Can we change feature H like this? “We can make these changes which get us closer to your vision, how does that work.” In this approach, you are trying to get a little closer to that vision/request while managing against your constraints (time/scope/cost/insert-here).
THINGS TO CONSIDER: Without breaking your constraints, how can you say yes? This helps build relationships and gets closer to the end product (quality/performance) that the end user wants.