Once a Main Sequence Star nearing the end of its life becomes a red giant, its temperature will rise sharply, and its volume will expand rapidly. It will quickly swell to 200 times its original diameter, engulfing planets close to its orbit.
However, this "quickly" is measured by the standard of the Universe.
For humanity, this process will last quite a long time.
But the Sun destroying humanity doesn't need to expand that much. Just a slight increase or decrease in temperature would be catastrophic for the Earth's Biosphere.
Once this sign appears, humanity might not have much time to prepare. A decrease in crop yields and climate imbalance would be enough to trigger a dramatic decline in the human population.
"Suppose, just today, you observed signs of the Sun turning into a red giant." Xiang Shan crossed his arms and closed his eyes, asking, "Then, given the authority to propose any plan you can think of, what would you do?"
David and Yawgmoth looked at each other: "This..."
