I've always found a well past prime, almost 40 year old Nunn with all his fire gone fighting all the way up to Cruiserweight underappreciated to some degree. He fought from MW to CW and beat great to respectable names in almost all of those divisions. Well into his 30's he went to Germany and fought Graciano Rocchigiani to a razor thin SD loss, a fight I think I had to Graciano by a point. Not bad consider Graciano was blatantly robbed against Maske and sparked out Michalczewski. Who else, and why?
Dingaan Thobela won the WBO lightweight title in 1990 and KO'd Glenn Catley in the last round of a fight he was losing to win the WBC super middleweight title in 2000. :good
Holman Williams - started at LW to go all the way upto LHW Billy Conn - LW-HW Cerdan - most of his career was as a WW, no one remembers that Carpentier - went from fighting for WW titles to HW titles (french/european) in 2 years. Charles/Moore/Patterson - MW-HW
You could be thinking about Ted Kid Lewis. I can track him down on Boxrec as having boxed between 116 and 162 fighting for the empire lightheavyweight title. He turned pro at 15. Incidentally he fought Carpentier as well at light heavy, Carpentier's weight that is 175.
ted kid lewis got paid a tanner(2 1/2p) for his first fight and a penny back for a cup of coffee !! legend
Joe Gans went up in weight to fight the original Joe Walcott. According to the newspapers Gans won going away, but it was an official draw. http://www.amazon.com/Joe-Gans-Biography-American-Champion/dp/0786439947/ref=pd_bxgy_b_img_a