You are still the same
Psalm (101 LXX) 102:27
Every good act of giving and every perfect gift is from above, coming down from the Father of lights, with whom there is no variation or shadow of shifting.
James 1:17
Jesus Christ is the same yesterday, and today, and to the ages.
Heb 13:8
Yes his character never changes in promise and morality, but he does change his mind.
As for 1 Samuel 15, it is the same message as in Numbers. God is not a man that he should change his mind, notice that it also says he should not lie. God doesn't act outside his morality. Man is right in his own eyes, the "righteousness" of man is unstable. If God changed his mind about his morality, then he would be a liar.
And Samuel said unto Saul, I will not return with thee: for thou hast rejected the word of the LORD, and the LORD hath rejected thee from being king over Israel. And as Samuel turned about to go away, he laid hold upon the skirt of his mantle, and it rent. And Samuel said unto him, The LORD hath rent the kingdom of Israel from thee this day, and hath given it to a neighbour of thine, that is better than thou. And also the Strength of Israel will not lie nor repent: for he is not a man, that he should repent.
Since Saul acted outside of God's plans, he changed his mind. He didn't make a mistake, but rather saul did, and he changed his mind and removed him from his place. The same with Numbers:
Except Ye Repent
By Dr. Harry Ironside
Pastor Harry A. Ironside
Chapter 11 - DOES GOD EVER REPENT?
In the history of Jehovah's dealings with the people of Israel there is perhaps no story more affecting than that of Balak's effort to induce Balaam to curse them when they were encamped on the plains of Moab. The faithless prophet who loved the wages of unrighteousness was eager to comply with the wicked king's request, but was hindered each time he attempted to curse the people, by the Spirit of God. At last he confessed his inability to do the thing for which he had been called to Moab and instead of cursing Israel he blessed them, and foretold their glorious future in such a manner as to stir the ire of Balak, and to move the hearts of God's saints to devout thanksgiving. He introduced the narration of the divine purpose concerning the tribes of Israel, with the remarkable words: "God is not a man, that he should lie; neither the son of man, that he should repent: hath he said, and shall he not do it? or hath he spoken, and shall he not make it good? Behold, I have received commandment to bless: and he hath blessed; and I cannot reverse it" (Numbers 23:19-20).
This is surely a marvellous declaration. It tells us that once God enters into an unconditional covenant with any people He will never call back His words. And He had definitely confirmed just such a covenant with Abraham. This was before the giving of the Law. The legal covenant they had a part in, and they failed to keep what they had promised. Only a few days later we read of the terrible sin of Baal-peor. On the ground of law they forfeited everything, and that covenant God Himself abrogated. But His covenant with Abraham was pure grace. He was the only contracting party. Whatever Israel's failures, He could not break His promise. He had bound Himself by an oath and He would not and could not repent, or reverse His decision. His attitude of grace through the promised seed would persist throughout the ages.
These are all in reference to his morality and his covenants. You showed his character never changes, now show that his mind never changes.
You can't. (Jonah 3:10)
The fact that he changes his mind proves that he never changes. He doesn't change his judgment, nor his mercy.