A changed commitment should be discussed before its cost reaches the other person.

Commitments change, and that by itself is not a failure. The damage comes in the gap between the moment you know and the moment they find out, because in that gap they keep planning, spending, and turning things down on the strength of something that has already shifted. By the time it surfaces, the cost has landed on them rather than on the conversation.

If something you agreed to is no longer likely, say so this week even though the details are unfinished. Give them the part you are sure of. Offer what you can still do rather than only what you cannot.