Maybe the people of the world won't be so quick to find a city and a country that has been transformed. "once" suggests a more immediate result. "when" suggests a less immediate result.
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Ikia"I feel this question is tricky. To me it looks like a question of logic, not of grammar. "Once..." is an adverbial clause indicating an unspecific time when some event happens. So, when a speaker say "once ...", he/she does not know exactly when the event happens.
"Once you've done your homework (I don't know when it is exactly), we'll discuss your pocket money"