A band called Frijid Pink did a heavy metal version of House of the Rising Sun in the 1970s but it might take some tracking down. How about the original Crossroads by Robert Johnson compared with the Cream version? Dirty Old Town started as a jazz song before becoming folk and then whatever you would call the Pogues version. Dylan's Hard Rains Gonna Fall rocked up by Roxy Music. Half of Led Zeppelins back catalogue started off as blues. Any help?