declare @reparented_node hierarchyid = '/'
declare @new_parent_node hirerchyid = (select path_locator from FileTable where is_directory=1)
declare @max_child_node hirerchyid  = (select max(path_locator) from FileTable  where path_locator.GetAncestor(1) = new_parent_node)
declare @new_child_node hirerchyid  = @new_parent_nod.GetDescendant(@max_child_node, null)

update FileTable 
set path_locator = path_locator.GetReparentedValue(@reparented_node, @new_child_node)
where path_locator.GetAncestor(1) = '/'