Well, I agree with the fact that A-Level exams are much harder to tackle, but from a university point of view it's just as important to refine your IGCSEs. Which subjects are you planning to appear in? What grades did you have which you want to improve on? Also, it sounds like you intend to do A-Level in one year. Bad idea. Trust me.
In general, you could give your IGCSEs this October/November, study for your A-Levels alongside and give a composite A-Level exam in 2013. Or even split it between AS and A2, really your choice. Good luck!