Shulchan Aruch/Yoreh Deah/363
Seif 1: We do not exhume a [buried human] dead body or bones – not from one respectable grave to another respectable grave, and not from one despicable grave to another despicable grave, and not from a despicable [grave] to a respectable [grave]. And it is not [even] necessary to say [that a dead body or bones cannot be moved] from a respectable [grave] to a despicable [grave].
But [in order to bury one] in one’s own [family plot], it is permissible [to move a dead body or bones] even from a respectable [grave] to a despicable [grave], because it is pleasant for one to rest next to one’s ancestors. And similarly, it is permissible [to move a dead body or bones] for the sake of burying someone in Israel.
And if [a dead body] is buried somewhere with the [prior] understanding that [the dead body] will [subsequently] be moved, it is permissible [to move the dead body] in any case.
And if the [dead body] will not be preserved in a particular grave because there is suspicion that idolaters will remove the dead body, or that water will enter [the grave], or that it is a found grave, one must [i.e., it is a commandment to] move the [dead body or bones]. (And there are some who have the practice to place earth from the Land of Israel in the grave, and this custom has on what to rely.)