Communicate Early and Often
Especially when changing expectations.
Communication is obviously a fundamental skill and that is never more apparent as when you are changing expectations. Often changing expectations means telling people that a feature will be shipped later than previously agreed upon or some desired functionality just won't be possible. Nobody wants to tell someone bad news and it can be hard to do.
But it's worse to put it off.
When you're changing expectations it is incredibly important to let people know as soon as possible and with full clarity. As soon as you see risks coming up, inform the people that will be affected by it. Nobody wants to be blindsided at the 11th hour when you tell them that the previously agreed upon ship date won't be hit.
And be clear about what you're saying. Avoid ambiguity and anticipate questions. Don't say things like "I'm worried about our delivery date". Instead say things like "Given our improved understanding of the complexity of this work, we should push the ship date back by two weeks". This is clear, gives context, and gives a description of the new expectations.
Don't avoid or delay important communications. Even if it's not what people want to hear, they'll appreciate hearing it sooner rather than later.