XSA-501 was about issues with v1 -> v2 and v2 -> v1 changes racing certain
other grant table operations.  Sadly a v2 -> v1 -> v2 change has yet
another effect which wasn't accounted for: During the first change, status
pages would be freed.  During the second change they would be allocated
again.  Hence across the full pair of change the addresses of the status
pages may change, rendering stale previously calculated pointers into the
status array.