简单地说,正确的表达是
He is no longer WHAT he was / used to be .
如果一定要用WHO的话,你必须在who的前面加个名词,比如说
-------- He is no longer the PERSON who / that he was / used to be
1 WHAT 强调he的属性,性格 和特点.
2Who强调he这个人本身.也就是说,他还是这个人,但性格特点或者处境身份不同了.