All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Muslim scholars have agreed that exhuming a grave is forbidden, because that violates the sanctity of the dead. In this regard, the Prophet(PBUH) said: "Breaking a deceased body’s bones is exactly like breaking them when he is alive." [Abu Dawood]. Moreover, Allah, The Almighty, Said (What means): "We have honoured the sons of Adam" [Al-Isra`/70]. Therefore, not exhuming the grave and not violating its sanctity is honouring its dweller.
However, if the corpse has fully decayed then it is permissible to exhume the grave and bury another corpse provided that there is a legitimate necessity for doing so. Al-Imam An-Nawawi said: "It isn`t permissible to bury a dead person in another`s grave unless its dweller has already transformed into earth, and nothing was left of his corpse; neither flesh nor bone." [Al-Majmou` Shareh Al-Mohazzab vol.5/pp.284].
In addition, the time that each corpse needs to reach a state of full decay varies from one area to another, and the people of each country are more aware of that. And Allah Knows Best.