In what instances may a body be removed from its resting place and be interred elsewhere?
Generally, a body may not be moved. However, see this article for details. Excerpted:
The Talmud Yerushalmi permits the removal of remains, even from a worthy to an unworthy place, that they may be buried with the ancestors of the deceased
In his code, Rabbi Joseph Karo adds other circumstances when exhumation may be permitted or is required:
A. It is a mitzvah for children to rebury a parent's body in Eretz Yisrael, even if the parent expressed objection during his lifetime to having his remains moved there.
B. When a burial site is unprotected from robbers or natural forces, it is permissible to remove bodies to locations that are not so compromised.
c. When a stipulation is made at the time of burial about the removal of the body at a later date.
D. When a body is buried in an available site, but without authorization from its owner, the body may he moved.