Porting is one of those staples of embedded development.
Find a like-for-like type platform moving from one NUCLEO to another typically involves less pin changes. The F4 and L4 have some architectural differences.
Make a scratch or donor project on the target board, get USART, SWV an basic functionality working. Mod the code in a way that simplifies the reception of the original code.
Use a good merge tool. Pull in code a section at a time. The code doesn't have to execute, at some level you are trying to insure the project continues to build, and address syntax errors/changes in the API, or pin usage. How much you bring in at a time depends on how overwhelming the changes are.
I migrated the LoRa code to L4 and F7 platform via NUCLEO boards, I also went from NUCLEO-64 to NUCLEO-144 within the same families, this allowed changes in pin/USART usage to be understood, addressed and refactored within a platform that was still buildable. For the NUCLEO in a given platform it was then an easier port to DISCO or custom boards.
