I think i have the same problem, imported all adresses from my mail program when i change the feed by mail property in the database newsletter plugin does not recognize the changed profile data.
The bulk action in Set preference (1) to feed by mail subscribers doesn’t work for my system.
only way to change the feed by mail property is manualy and x 4000 is not a fun job.
That function is used to move the feed by mail subscriber to a preference. It was made for another extension. But your code is correct, only I’ll add the specific action to subscribe every user to feed by mail.
At least I’m looking inside newsletter 3 that should have the same code, or are you taking, as I think, the code from newsletter pro? May be there is a wrong action labeling there.
i know that this is an old post but how do i make this happen in the newsletter pro, so i dont have to go through all subscribers manually and turn on the feed by mail