Generate reversed operations for changes, that require full rollback and creation.

def _generate_reversed_sql(self, keys, changed_keys):
        """
        Generate reversed operations for changes, that require full rollback and creation.
        """
        for key in keys:
            if key not in changed_keys:
                continue
            app_label, sql_name = key
            old_item = self.from_sql_graph.nodes[key]
            new_item = self.to_sql_graph.nodes[key]
            if not old_item.reverse_sql or old_item.reverse_sql == RunSQL.noop or new_item.replace:
                continue

            # migrate backwards
            operation = ReverseAlterSQL(sql_name, old_item.reverse_sql, reverse_sql=old_item.sql)
            sql_deps = [n.key for n in self.from_sql_graph.node_map[key].children]
            sql_deps.append(key)
            self.add_sql_operation(app_label, sql_name, operation, sql_deps)