If you brought the bottom ~2 billion up to the median global living standard, the total human population would begin to contract due to the reduction in new births brought on by the higher standard of living. If you reach an income level on par with poor Eastern European nations ($10k to $15k), you start to see a decline or flattening in population.
With accidental deaths, and the huge drop in new births, it wouldn't take very long - relative to the new life expectancies - to cut the population of the planet in half.
If all people stopped dying at 65, and say lived to 400, the wealth accumulation would be substantial for most of the poorest (which currently reside in China and India). Their standard of living would skyrocket, as they'd have perhaps 300 good working years. Those 1.5+ billion poor living in China and India would also likely live long enough to see the conditions of the poor in said countries improve immensely. Not to mention the remarkable breakthroughs in technology / medicine that would occur during their lifetimes, a further multiplier lifting them up out of current levels of poverty.