You missed the point. He 'already changed (or rather guided/made the past He desires to exist)' is correct.
However, If God created us, He can and easily manilpulate the universe to reflect a different existence. Open Theism is unintelligible on this particular, sorry, not logical at all. You'd say that the alternate past still 'existed' but I (and many others with me) am saying "nope." Pure assertion. As such, we are simply arguing to argue. There is no point in an assertion. It is my strong belief if you erase something as if it never existed and there is absolutely no way to prove it existed, then it doesn't exist, period. So rather, "God is completely able to change the past," against your weak assertion. Anybody that reads in their Bibles "God Almighty" (Omnipotent) and believes it will disagree with your "asserting for no other reason than to support a preconceived (erroneaus) notion/theology)."
If you believe this for any other reason, please let me know. I find it completely untenuable and not based on any kind of logic but to support an erroneaus preconception. It is, imo, against the clear teaching of Scripture. "Almighty" means able to do all things doable (not that I, a mere human could limit Him by my or your limited conceptions), and changing the past is certainly, unequivocably, doable by sheer power.