There is change which is explained well, and identifies the sort of constancy in "the mission" that leaves employees with a clear sense of roles and responsibilities, and there is change which undermines role clarity, and consequently undermines engagement. There is change which assists someone to do their work better, or achieve more, and then there is nonconsultative change which impedes someone doing their job better. My experience with organizational change is that those who impose it think in terms of only those segments of the organization they are most familiar with, but don't consult much beyond that.
The dedicated within any public bureaucracy are committed to a particular established vision of the mission of their organization. That vision lets them know not only what is to be achieved, but how the parts work together to achieve it, and how to get it done. Any successful change will leave that vision intact, or at least translate it to the new environment so that it feels, for the employee, that only the superficial elements are different, and the fundamentals remain intact. Some employees will be able to figure that out on their own. The majority, though, will not, and will require direct contact by management, or something that feels equally authentic, and can persuade them that what they've dedicated themselves to is still respected.
For some employees, being consulted is vindication, and what they treat as "recognition". So when the organization changes, what value will their accrued knowledge have? What will permit them to feel that they still have value within the organization? How do they know they will continue to be consulted? Until that is resolved, do not expect their full effort.