Man says to his wife: "You haven't changed a bit in the 20 years I've known you."
Does he mean that she has never changed her mind, or that she has never changed in character (or something else)?
Since we see numerous examples of God relenting, then how can one say He does not? Malachi 3:6 must, therefore, be speaking of His character. In Malachi 3, He speaks of how He will treat His chosen people as He always has, both in terms of judgment and in terms of mercy. And just like He did with them centuries prior to that, if they turned to Him, He would return to them.
Even from the days of your fathers ye are gone away from mine ordinances, and have not kept them. Return unto me, and I will return unto you, saith the Lord of hosts. But ye said, Wherein shall we return? Mal. 3:7
He can
do or
not do...but that doesn't change Who He
is.