They are both mainly used as enhancers I thought, if I don't have any dextrine malt I use one or the other for head retention myself. I really don't see why you couldn't sub it, only thing I personally would see myself is the flaked wouldn't give you that added toast flavor for a bitter that the torrified would provide. But maybe you could toast it in the oven yourself to add it? I'm curious to hear what others would think though, I'm always looking for more info on grain subs.