This will be managed with communication skills.
If you produced a scope statement that was approved by them and you really know that the scope was like 1+1=2 and now is like 1*2=2, try to evaluate if the goal of the feature produce same results and if they are willing to accept it if this does not change all the process. It will depend on how your product or service is delivered. (easy to fix?)
If you believe, after an analysis, that your on track with the project and it does not represent a huge impact or rework, try to evaluate if you can assume it with disclosure that this will be handled as an exception.
If it is completely risky, try to make reason the customer and try to deliver a release v1 and then v2 on a later phase with another scope and budget.
If the change was produced by a huge reengineering by the customer then there is no choice that they should assume this change with the impact in time and cost. If you maybe can prioritize some features and you can still deliver basic functions, you may produce an outcome.
Try to negotiate first before to make a change or go up in hierarchy.
If there is no chance to close a deal then produce a formal change request since your baseline was modified and make validate by customer formally specifying how much time and money will cost.
If there was not scope declared even if these come from fixed contract, you jeopardized your project. It will cost you money and time. You should have stated the limits/scope for each requirement that customer requested from SOW.