UX research after functional requirements

I'm working as a UX Specialist and need to plan/build the processes involving UX. Perhaps any tips on the following:

1. If functional requirements are already determined and project costs are estimated based on the requirements, what if in the user persona research, competitor analysis or user journey mapping, you find out that some additional functionality is needed? Would you go back and add it to the requirements? Then do the estimation again, then get the approval from client?