I am not sure if there is a defensible Schelling point in this particular Sorites. Possibly whether your actions cause a self-sustaining reaction, or something.

Volunteering in poor areas can make a difference between life and death for a few people. If you end up achieving Mother Theresa-like prominence, with lots of followers saving lives out of proportion to your personal efforts, then maybe it's close to changing the world.

Or maybe you just have enough leverage to achieve a significant effect, like Bill Gates planning to eradicate malaria completely in a decade or two. But most of us are not in that situation, so we have to rely on others to create a "world-changing" amount of leverage.