Whatever you decide it must be Big. No small event can change such a person. Do they have a near death experience? Do they or a loved one have health issues. Does your character climb a mountain to prove their worth and have an epiphany? You get the picture.
In the beginning of your story you must invest some time establishing their Greedy nature and perhaps all the luxury they have because of it. I'd say no more than one chapter. Then bang, perhaps their investments go south (slang for they lose everything) or perhaps they realize that the homeless woman on the corner is suffering do to circumstances beyond her control.Thus they get involved with charities and take their business savvy into a different arena.
An alternative to this situation is when another character or a group of characters perceive your hero/heroine to be Greedy when they are not. You are still dealing with the same character flaw and you must establish the reasons others think this. Once again, you go Big with your logic to paint a picture that is false. Then you must go Big to change the minds of those around your hero/heroine.
In every example I used you will find books, movies and television shows. However, only you can write your story, in your voice and make it original.
Any thoughts? I a movie, book or television show come to mind? I hope this was a little help to you.
Picture Link: Wikipedia