How does macbeth's character change from valiant soldier to murder and tyrant?Macbeth begins the play as a valiant soldier however as the play goes...

Macbeth, as you said, began as a superb soldier and wonderful warrior, but becomes a bloody tyrant and degenerates with the course of time. And the chief reason behind his degeneration is his ambition.

Once a person becomes a murderer and obsessive to achieve power, then to hold on it, he can do anything for the sake of power. His gluttony makes him paranoid and destroys all his good qualities gradually. it becomes difficult for him to come back from the path of hell he chooses for himself. Macbeth in Shakespeare's Macbeth, faces a very similar situation. He says: "To be thus is nothing, / But to be safely thus.", and in order to secure his kingship, he continues assassinating on and on. Later he utters, "I am in blood/ ... Returning were as tedious as go o'er." (act 3, scene 4). so, a brave warrior and an obedient royal employee becomes a 'butcher' very soon because of his inner devil. Here, we find a similarity between Marlowe's Dr. Faustus and Shakespeare's Macbeth.

Macbeth, as my reading of the play suggests, was already an ambitious man while his wife was greedy. What he needed was just an instigator, a spur. The witches' prophecy or equivocation worked like the spur. It is evident in the fact that, when the witches were uttering their prophetic statements, he had Banquo with him, and it was stated that Banquo's progeny would be successor to the throne. Still, it was only Macbeth, the Thane of Glamis, who took the "weird sisters"'s statements seriously. Banquo, unlike Macbeth, does not believe in the witches' prophecies blindly. Rather, Banquo is shown to say: "The instruments of darkness tell us truths/ Win us with honest trifls, to betray's/ in deepest consequence..." when asked by Macbeth: "Do you not hope your children shall be kings...?" (Act 1, scene 3). It is Macbeth's inner evil which, spurred by the witches, became more gluttonous. Later, his wife helps to the full growth of the ambitious tyrant inside him who could do anything for holding on the power.

So the reply to what you've asked: "did Macbeth already began planning to be king before the witches?" will be that,  Macbeth is not directly shown to be plotting to be the king, but it is clear in Lady Macbeth's utterance to herself in act 1, scene 5, that both were aiming to reach the goal when she says: "Thou wouldst be great, /... / To have thee crowned withal".