Bob Sapp- ex NFL, now K1 star.
Jim 'blackbelt Jones' Kelly switched from eastern MA to queensbury rules, though I am not sure of his success.
Mike Tyson raced pigeons before boxing (I know thats a stretch

)
To be honest, with the state of the Heavyweight scene at the minute, I am suprised more 'big' athletes dont throw their hat in the ring- ice hockey, NFL, rugby, and of course the various kickboxing styles (all of which have much less prize money), could all offer at least a handfull of people with the physical potential to beat the fat sweaty blow-hards that pass themselves off for heavyweight contenders these days.