Game mode is not the same as game engine ( Frostbite in Battlefield for example ). Adding an additional customizable game mode ( championship in this case, it can also be adding arcade mode, 2p split screen vs etc ) would not require total overhaul of the game IMO. Games nowadays would be modular in design, with each game mode like a part of bigger body. Similar like devs who add online mode into the game that was offline only originally ( Konami with MGS V TPP + MGO ) When MGO went online, players have to download update data to make MGO functional.
Patch means new build : new executable, new assets ( added cars, track, liveries ) when DLC included, new physics build ( updated tire data, car model data etc ), bug/error fixes, stability updates, adding a new championship game module with customization would not be very different undertaking, I think, anyone here have done game development ?