An estimate is a claim about the world, and the world moves. What usually happens instead is that the number stays fixed until pressure arrives, which means it is being adjusted by whoever pushes hardest rather than by what is true. Everyone learns that, and the result is a system where complaining works and patience does not.

When something you promised shifts, update the number the day you know, before anyone asks. Say what changed and what it now looks like. It is an uncomfortable message to send early and a far worse one to send late, and the person on the other end can plan around the early version.