If there isn't any more noticable rust, that means that other areas of the roof are fine. If you try to redo the WHOLE roof, there is a chance that you will cause more rust problems down the road.
If there is any more rust that hasn't presented it self, then you just repeat what I said before again. And if you want to go ahead and redo the whole roof
then go ahead, yo see how difficult it is to even the roof.