This is an interesting question, because it is possible to argue that Pip's expectations have not actually "changed" at all, in the sense that all of the foreshadowing that Dickens has carefully planted has come to fruition, and we see that the association with crime and with illegal activity that has accompanied Pip's life from the opening chapter, when he is forced to steal for Magwitch, comes full circle and he realises that his "great expectations" are based on nothing more than dirty money emerging from a convict.
However, on the other hand, Pip has believed that his great expectations are thanks to Miss Havisham, and that part of those expectations involves a happily-ever-after marriage to Estella. With the realisation that his wealth has a very different source, poor Pip is left to realise that his hopes and dreams of being intended for Estella are vanished, as his response the following morning shows:
When I awoke without having parted in my sleep with the perception of my wretchedness, the clocks of the Eastward churches were striking five, the candles were wasted out, the fire was dead, and the wind and rain intensified the thick black darkness.
This revelation has signalled both to Pip and to the unobservant reader (if they haven't already realised) that this will not be the fairy tale that Pip thinks it will be, but rather a somewhat more warped reality which plunges Pip into darkness as he realises the truth about the source of his expectations and where that leaves him. Note the pathetic fallacy in this quote: the weather mirrors the feelings that Pip himself is experiencing.
One day the convict whom Pip helped in the marshes comes to Pip after several years in London. The Convict tells Pip that he is the real benefactor of Pip. It is he who has brought him as a gentleman. Pip gives him shelter. His expectations appear to dwindle away. He is somewhat heart-broken. He feels that Miss Havisham has tried to make a mechanical model of him and that Estella is not designed for him. Thinking that Miss Havisham to be his benefactor he politely obeyed to her words, for the love of Estella. But when he came to know that the Convict is his benefactor all his expectations of Estella shed down with a broken heart.
Since Chapter 8, Pip has been expecting (or rather hoping) to move up in life and become worthy of Estella. When he received his money from his unknown benefactor, Pip automatically assumed that this was Miss Havisham's way of helping him become a gentleman, and thus, marry Estella. In Chapter 39, Pip discovers that his benefactor is none other than the convict from the start of the book. It is at this point that Pip realizes that his expectation of someday being worthy of Estella will never come true. He has money, but it is ill-gotten money in his opinion. And his assumption that Miss Havisham wanted Pip with Estella has been proven wrong. It is at this point that Pip realizes that the one thing he has been working towards (Estella) will never be. Pip also realizes that in his pursuit of Estella he has trampled on his best friend Joe. Chapter 39 essentially crushes all of Pip's expectations of being a gentleman. It is after this chapter that Pip realizes that no matter how much money he has, he will never be worthy of someone like Estella. It is also at this point that Pip starts to realize that his expectations may have not been worth all he has sacrificed in life (friendships, home, his true self).