And presuming a species that built a generation ship doesn’t have the ability to handle these environmental changes (either through “fixing” the environment or the genes). And there are two migrations here. I’m not seeing much targeting the earth to generation ship migration directly here. But, they’d all die in free space so the ship is an environmental “fix” and they may need a genetic “fix” to handle things like long term exposure to lower gravity, or some quirks of centripetal gravity.