I entirely agree that this is definitely an important aspect of Open Theism's theology proper but I'd stop short of calling a primary premise. The primary premise(s) of Open Theism are God's qualitative attributes. It is the fact that God is loving and just (and in all ways righteous) that stands as the primary premise(s) upon which the idea that the future is open depends.
Of course, that's not far away from the idea that God is free. My point here is only that the argument goes in the other direction than your statement would imply. Open theists generally begin with the conviction that God is genuinely personal, loving, righteous, relational, responsive, etc. From that, they argue that His relationship with creatures must involve real interaction: human choices genuinely matter, prayer can make a real difference and God can respond differently depending on what creatures do. Which is to say that God's ability to change His mind is down stream from the quality of His character (logically speaking).
Having said that, I suppose it does depend on just what argument you are building because if God were not free then He could not be moral. In that sense, the fact that He is a free personal agent is logically prior to moral agency, but still, that isn't usually the way the argument goes. We typically start with the premise that everyone agrees with and argue from there.