I would suggest that there are a couple of ways to look at "the life changing event" that George undergoes.  On one hand, I think that one major "life changing event" would have to be when George tells Lennie to go jump in the lake for fun and Lennie does so out of loyalty and devotion to George.  This is life- changing because it shows George just how loyal Lennie is to him and how much Lennie depends on George.  It was at that moment that George recognizes that his destiny is linked to Lennie, and that he has to be responsible for Lennie.

Naturally, the "life changing event" that most will immediately consider would be his killing of Lennie.  I think that there is little insight offered by Steinbeck after this and it might be deliberately so.  Steinbeck does not reveal how George was changed by it.  Yet, it becomes something that George recognizes that he needs to do.  Having witnessed Candy's own guilt about not killing his dog and letting Carlson do it, George recognizes that he must take Lennie's life if he has love and devotion towards him.  It is for this reason that he ends up doing so.  George is changed after it, needing Slim to steady him and share a drink with him.  Yet, it is clear that George recognizes that his life, to an extent, has come full circle.  From the time when he told Lennie to jump in the water and when he rescued him,  George has now moved to a point where taking Lennie's life is the only humane option in a world that is predicated upon inhumanity.  It is here where George stands, poised between the polarities of two "life- changing events."