Thinking a bit more, there is an issue here. I suppose it is preferable to have the following behavior: if an user exports the GMF of a previous computation and gets the value in a given order, after the migration it should get the values in the same order (I am saying inside the XML file). The RENAME ensure that the ordering is preserved with no effort at all. Instead the following

ALTER TABLE hzrdr.gmf_data ADD COLUMN task_no INTEGER NOT NULL;
UPDATE hzrdr.gmf_data SET task_no=ses_id;
ALTER TABLE hzrdr.gmf_data DROP COLUMN ses_id;

would be terribly slow since the UPDATE would involve billions of rows (the table gmf_data is the biggest table we have).