Your film has no ISO. The ISO is an organization, not a description of your film. They determine the standards for how a film's speed is defined and determined, as well as numerous other international standards. So, ISO film speed is set in stone, because it has the "ISO" attached to it. Since they name their criteria and processes that determine a film's speed, when you change these things or use different criteria, you no longer call it "ISO" film speed.
Re rating your film changes only one thing: exposure. Since a film rating is simply an input into your cameras exposure meter, changing this could not possibly change the actual sensitivity of the film; only what the meter does. So, since your camera thought you were using 100 film, but your film's speed is actually 400 ISO, your film is two stops more exposed than it would have been if you had set the meter at 400. Since it is negative film, it will be ok, though you may want to underdevelop the film 20 to 30 percent to make it easier to print.