During the project implementation, change management is crucial because if it does not properly perform change management guarantees great misery both during the project and after the project is completed.
Firstly, it should be very clear to all stakeholders and project team members what should be delivered and the baseline needs to be clear. The challenge starts when changes are being raised and to keep track of the changes on the baseline! Here lies the great danger in 'customer service' and 'we arrange this among us informally'. It also is possible that change management on the other hand is a huge bureaucratic affair. The Project Manager, in consultation with stakeholders determines how best change management can be applied in this particular setting.
Basically, every change is analyzed and the impact of the change determined, in particular lead time and costs.
A change may also affect the contract. In this case you as a Project Manager need to be extra careful and you have to consider additional lead to your account (partly because then additional sections of both client and contractor as the legal departments are involved in the project).
The change is implemented only after approval by the right people at the client. Think always of a 'roll-back' scenario especially if it is a business critical environment (in the case when the project has impact on the production environment). Normally a 'DTAP street' (Development, Test, Acceptance and Production environments) is being used before a change is actually rolled in a production environment. However, it will not be the first time that the acceptance environment is not quite identical to the production environment with the result that this 'fall over'.
Finally it is important that the project team not to inform, but also to involve changes, especially when it comes to impact analysis.