Simply dragging and dropping a 100% save file usually won't work because saves are tied to a specific Xbox User ID (XUID)
To guarantee the game records your progress after swapping, change one minor visual detail (like a window tint or rim colour) and then manually drive into a Player House Festival Site to force a save. Disable "Quick Resume" (Xbox): Forza Horizon 5 100 Save Game BETTER
If you want to experience the 100% save without risking your main profile, create a secondary Hotmail account . On PC, you can use Steam Family Sharing Simply dragging and dropping a 100% save file