How to change the Controller+Action if certain criteria met

I have a base controller, and in it I am loading the user model. If the model has some certain flags active, I want to change the action.
e.g. User is visiting forum/thread means forum controller and thread action. But in the user model, the "is_locked" flag has value "1", so I want the CI to use the "account" controller and "locked" action, but I don't want the page to redirect to account/locked.