If we simplify it to Chinese population growth, the growth rate is obviously going to change when increasing the 1 child policy to a 2 child policy.
Why? People had 2 children under the 1 child policy, others had none. Some had more. The effect of the One Child Policy on fertility is a highly debated topic with no clear answer.
Care to explain why it won't have any effect rather than just being uppity? If 400m people weren't born due to this policy, surely its removal will mean that future growth will change as a result?
The widely quoted figure of 400m not being born due to the One Child Policy is the result of a misunderstanding in the media. When the policy was introduced, China's TFR was already approaching replacement level. The previous decade of trauma had taken care of that, and the continued development of living standards throughout the 70s and 80s ensured it would continue to fall.
The trouble with projecting populations is that population momentum is a fickle mistress. Future growth may change as a result. It may indeed increase. However, it may also decrease. The fact that the government no longer wants to fine or punish those who had more than 1 child doesn't mean that every female of fertile age in China will suddenly increase their fertility, just as the fact that the government suddenly decided it wanted to fine and punish those who had more than 1 child didn't mean that every female of fertile age in China suddenly reduced their fertility.
A micro-level process as complex as fertility has far more determinants and variables, despite what governments (not only China) like to think.
In many examples, the biggest determinant of fertility is female education. Who'd thought that teaching women and allowing them to expand their horizons would mean that they didn't want to bear as many children?
There are some who make the point that the One Child Policy could have actually increased fertility for some women, and if you think about it, that makes sense. If children become valuable (because the government says you're only allowed one), then of course couples will think more carefully about having children. Those that hadn't planned to before may decide to, to take advantage of a surplus of men, for example.