Purity: Exhumation and Reusing Graves
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 612:
It is not permitted to exhume a grave for another corpse to be buried there unless the grave is very old, the first corpse has completely decomposed, and the act would not necessitate sinning, such as encroaching on the right of another person.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)