Lotte,
The project charter is created in the initiating stage and needs to be broad enough so it does not need to be changed as the project evolves. If the project charter is changed, the project sponsor must approve those changes. Depending on the changes, it may also be necessary to revisit parts of the project management plan. For example, if something about the overall objective needs to be changed then it would be necessary to reevaluate the project plan in light of those changes. Hope this helps.
Mark