How late is too late to change project scope on a failing project?

Project launch. Any changes to scope after starting a project will bring us back to the drawing boards.

Scope or WBS alongwith Cost and Time are Project Baselines and changes made in any one of them can impact a change in the other parameters and hence it is not advisable to make changes in Project baselines during later stages of the project.

Once the Project Management Plan is finalised and Buy-in / Sign-off / Formal Acceptance from all Stakeholders is obtained and it is only then that the execution stage is started. Hence-forth changes in the Baselines is not acceptable.

However incase the change is unavoidable, the impact of the change has to be detailed and the Change Request has to be approved by the Integrated Change Control Board or the Sponsor.

Once approved, the Project Management Plan has to be updated and configured to accomodate the change.

You can thus observe that the matter is too complex and it is thus advisable to deny changes in Project Baselines once the project is under execution stage.      

The key to successful projects are the foundations which has been built by timely plans and research beforehand. Therefore a quick minor change which might comply to its foundation is still acceptable however a primary alteration can't be made as it would surely mess up the base that has been made but if you have the guts, as it calls for necessity why not, just be sure to be ready to take the responsibility to what is waiting beyond.

By having a failed project, we already have the variales that caused the project to fail, with these variales corrects the scope to be able to be roade the project.