There are more Blacks than Whites on earth. Imagine if even 1% offered to go under racial surgery? That's a giant boost to the White population right there.

Or look at cases when racial lines have been blurred. Such as nations like Brazil with its half and half people. If the existing mixed people wanted to become White, than virtually the entire country would soon follow since they would give birth to 100% European offspring that breed with the other 50% White people.