The Code Conversion Utility changed the behavior of Byte Array To String by wiring an extended ASCII encoding enum constant to try and preserve the original behavior of the application.
LabVIEW NXG does not support flattening or unflattening data to a binary string because LabVIEW NXG uses UTF-8 string encoding. LabVIEW uses ASCII encoding, so the Code Conversion Utility automatically replaces instances of Flatten to String and Unflatten from String with Flatten to Byte Array and Unflatten from Byte Array, causing the flattened data type to change from a string to a byte array. This change may lead to broken wires on the diagram.
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.