In the case of Singapore it would be just the opposite:

Singapore was and is build on the back of immigrants. Replacing ourselves with kids instead of immigrants would be the big change.

Whereas a society replaced by children...

Whereas a society replaced by its own children maintains continuity of community, nation, or broad grouping of people having common traditions, institutions, and collective activities and interests

What about those societies that have traditionally replaced themselves via immigrants? Moving to a replacement via kids would be the change for them.

Plenty changes between generations. Still not the same society.