All I am saying is, if you POR-15 over the paint, your not guarnteed of anything, My assumption was: The POR-15 was being used to "rid" rust, and keep it from every starting. If you leave the paint on... sure the POR-15 will definitely adhere to it, but like someone already stated, who's to say, it want come back under the paint that was left.
As far not doing anything: I am only saying, your better off waiting till you got the down time available to do it right, than doing it ... hoping, and then later have to strip a layer of POR-15 that was put over paint, that was starting to rust under it, but hadnt showed through the paint yet. Then you got a big JOB getting all that back out. And for what the POR-15 cost's, I want it on something that I know will be eternal and will last forever, and putting over previous paint, regardless of how much you sand the paint is not a reassuring thing for me.