Well, Macbeth changes more or less as soon as he has done the deed. He comes back in to tell his wife that he has killed Duncan, but seems unsettled, maddened, unsure of where he is:
Whence is that knocking?
How is't with me, when every noise appals me?
What hands are here? Ha, they pluck out mine eyes!
Macbeth seems similarly unsettled by the murder in the next scene, in which he reveals he has - impulsively, without checking with his wife - murdered Duncan's grooms, so terrified he is of being discovered.
And then we learn in Act 2, Scene 4 that he has been crowned. And here is where, I think, there is a real change in Macbeth. His language becomes more muscular, and, though he is still hugely neurotic, he now is also hugely powerful. He orders Banquo's murder, and, though he is maddened again at the banqu-et (thinking that he sees Banqu-o's ghost) he regains his resolve after the apparition scene enough to order the murder of Macduff's children (though we don't see him do this).
Perhaps the best answer though, would be to look at the Macbeth at the end of the play. He is drained, weary, cynical and completely sure that his life is worthless. He knows, too, what he has missed out on:
...My way of life
Is fall'n into the sear, the yellow leaf,
And that which should accompany old age,
As honor, love, obedience, troops of friends,
I must not look to have.
The murder of Duncan is a mistake that costs Macbeth his life - and his quality of life.
Since Duncan’s murder, Macbeth has become ruthless and more set on murder than before. His greed for power influences his decisions to murder. He still deeply relies on the prophecies of the three witches and, therefore, decides to rid himself of the next obstacle, Banquo. Macbeth feels his crown is not enough. He wants the lineage that was prophesied to Banquo:
Upon my head [the witches] placed a fruitless crown and put a barren scepter in my gripe, / Thence to be wrenched with an unlineal hand, / No son of mine succeeding
Macbeth’s guilt seems to consume his thoughts as he sees Banquo’s ghost at the party. He soon becomes irrational and almost gives away his ghastly secrets when he sees the ghost sitting in Macbeth’s place:
Which of you have done this?... / Thou canst not say I did it. Never shake / Thy gory locks at me
In Act IV Macbeth visits the witches and is informed that he needs to be watchful of Macduff. Again Macbeth's murderous thoughts are evident as he plots to kill Macduff's family. Now, his lack of guilt shows the ruthless savage he has become. He needlessly kills Macduff’s wife and son.