The Code Conversion Utility replaced the Expression Node and the Formula Node with the C Node because this version of LabVIEW NXG does not support the Expression Node and the Formula Node.
Review the contents of the C Node for correctness. Replace any unsupported methods with supported methods. Remove the REVIEW tag once you are satisfied with the contents of the C Node.
If the previous suggestions don't restore the functionality of the affected code and the code isn't essential to the core functionality of your application, you can remove the portion of code. If the code is part of the core functionality of the application, you can use the VI Interop node to call your existing code, provided that the new code organization meets the architectural goals of your application. If neither solution meets the requirements of your application, do not migrate to LabVIEW NXG at this time.