clowns are usually only compatible long-term in same-species pairs; in larger tanks, people can sometimes get by with keeping multiple pairs, or entire clutches, with some success, but a 75g is just far too little room for more than a single pair to be able to live peacefully. it sounds as if you have a pair of black-morph A. ocellaris, as well as a single standard-morph A. ocellaris, and the standard morph is giving the two newer black morphs trouble? and you want to swap your current standard-morph out for a different one?
if thats the case, you'll end up with a similar problem, if not one clown pushing out the others, than two pushing out one. boot the trouble maker, and let the other two live in peace. or, if you want both color morphs, boot the trouble maker and then swap one of the blacks for an orange.