Can i appear for cs exam?

Can i register for CS foundation now for writing in december 2013? and by the way i haven't given my b.co exams this march,which is my first year.so.........can i cancel b.com and join CS,coz i'm prepaing for ipcc too! i wanna do CA and CS instead of CA and B.com! if at all i can,can i go for ignou after ipcc? god!!! i'm so much confused!! this might seem a li'l too much but....pls clear my doubts! i know dat we can do only one course along wid CA and dat too wid institute's permission!! but,i donno how we can choose and how the exact rules go!!