Doubt regarding articles

Hii people,

 I've passed IPCC both groups in my first attempt in november 2010 with 61%. I joined a small firm in March 2011 but would like to switch to deloitte after my CS exam next month. . I heard that within the completion of first year of articleship, i can transfer freely.. Is it true? Plus, will big four hire articles midway through?? Pls do let me know..

Thanks & regards,

Shreenidhi