So before upgrade u reinstalled windows on old mobo? Try reinstall windows on new mobo, all drivers will be deleted and reinstalled automatically by windows.
https://www.notebookcheck.net/Intel-UHD-Graphics-620-GPU-Review-Benchmarks-and-Specs.239936.0.html
With i5-8250u on 720p low fortnite 39 fps. It will be hard to play. Don't worth that. But if 25-30 fps is normal for u, u can play