Firstly, admitting a new member into the UN as a recognized state was not what happened with China.
For China, the seat and the country didn't change... "China" held the seat since the very founding of the UN. What changed was the recognition that the PRC had succeeded the RoC, as the sole representative of China. Similar to how the Russian Federation succeeded the USSR.
United Nations General Assembly Resolution 2758
No new country was admitted to the UN, they only changed the representatives of a country that was already an existing member.