json obj1 = {"key with space" : 10}
json obj2(obj1);
# .change_key not a real function
obj2.change_key("key with spaces", "key_with_spaces");

json obj1 = {"key with space" : 10}
json obj2(obj1);

obj2["key with spaces"] = obj2.at("key_with_spaces");
obj2.erase("key_with_spaces");