Major changes in project scope or budget would warrant an updated project charter. These would be changes which exceed the reserves and contingencies set aside to manage risks and smaller changes.
This would be especially important if a Statement of Work (SOW) is serving as the project charter as it may be a legal document as well (e.g. part of a contract).
When it is appropriate, the project manager should bring up the change with the project sponsor who would be responsible for making the updates and working with the PM to communicate them.