If the future is set, either by God’s foreknowledge or by God’s decree, then there should be no such thing as the future being changed. Either God would have always known the new events would happen (making second set of events fixed) or God would be making the new events happen (making those events fixed). So if God knows the future (like watching a movie), there should be no such thing as someone who God says is going to die then getting 15 extra years as a result of his prayers. But we see just that in 2 Kings 20 (paralleled in Isaiah 38):

2Ki 20:1 In those days Hezekiah was sick and near death. And Isaiah the prophet, the son of Amoz, went to him and said to him, “Thus says the LORD: ‘Set your house in order, for you shall die, and not live.’ ”
2Ki 20:2 Then he turned his face toward the wall, and prayed to the LORD, saying,
2Ki 20:3 “Remember now, O LORD, I pray, how I have walked before You in truth and with a loyal heart, and have done what was good in Your sight.” And Hezekiah wept bitterly.
2Ki 20:4 And it happened, before Isaiah had gone out into the middle court, that the word of the LORD came to him, saying,
2Ki 20:5 “Return and tell Hezekiah the leader of My people, ‘Thus says the LORD, the God of David your father: “I have heard your prayer, I have seen your tears; surely I will heal you. On the third day you shall go up to the house of the LORD.
2Ki 20:6 And I will add to your days fifteen years…

So God tells Hezekiah that he is going to die (which he does not). Hezekiah then petitions God. God then adds 15 years to his life. How does this happen with a set future?

To top it all off, the first thing that Hezekiah does is bring in emissaries from the Babylonians (showing them his wealth), and promptly getting cursed:

2Ki 20:17 ‘Behold, the days are coming when all that is in your house, and what your fathers have accumulated until this day, shall be carried to Babylon; nothing shall be left,’ says the LORD.
2Ki 20:18 ‘And they shall take away some of your sons who will descend from you, whom you will beget; and they shall be eunuchs in the palace of the king of Babylon.’ ”

Hezekiah’s response: “Don’t care; it won’t happen to me.”

2Ki 20:19 So Hezekiah said to Isaiah, “The word of the LORD which you have spoken is good!” For he said, “Will there not be peace and truth at least in my days?”