It depends on who you mean by "US commanders in the Pacific" but for one example, Curtis LeMay, was not told until late March 1945. That is fairly "operational" (though LeMay did more than just that), but gives you an indication of how late it was. Admiral Nimitz was not told until January 1945. He was told by Fleet Admiral King, but I don't know exactly when King knew. The impetus for this early-1945 "informing" was, of course, because of operational needs for deploying the weapon (building up facilities, etc.).

I don't know off-hand when Admiral Leahy was told, but my recollection is he would have known a lot earlier. There were other people at higher ranks of the military, like Marshall, who were aware of this project from the very beginning.

There isn't really any indication that the atomic bomb played any real role in the strategic planning until after the Trinity test in July 1945. Until that point, it was treated as something that might play a role, but there were significant uncertainties involved (and it is clear that some, like Admiral Leahy, were dubious it would even work).

Even after Trinity, the main planning was about when to drop it. It wasn't about whether to change anything about the existing plans. The plan was always to bomb and invade; it was never seen as a "trade off" at the time. Only after the bomb was "successful" in apparently leading to Japanese surrender did it get reframed as having been done as a sure-fire way to avoid invasion.