bmaupin's Save Game Editor allows you to change several settings of a saved game, see the following for more detail: https://gaming.stackexchange.com/questions/73480/is-there-any-way-to-change-victory-conditions-with-the-game-already-started/273907#273907

There is actually a web version to make it easy to edit the save game: https://bmaupin.github.io/civ5save-editor/