Drupal 7 file attachment issues after upgrade

In Drupal 6 I used the public file method, combined with the Private Upload module, for which there is no Drupal 7 version. After the upgrade I wanted to use Drupal's private method. After the migration I entered the file locations under admin/config/media/file-system:

Then I proceeded to admin/structure/types to enter the values for each data type. However, in the File attachments field settings section a warning was displayed: These settings apply to the File attachments field everywhere it is used. Because the field already has data, some settings can no longer be changed. The radio buttons for private/public were grayed out:

That makes sense, since many attachments were present from D6. It doesn't mean that all private attachments from Drupal 6 were now public, but new attachments would have been. Here is what I did to work around that. It didn't make me proud but it was relatively quick and easy.

First I temporarily modified the Drupal core file modules/field_ui/field_ui.admin.inc and replaced the $has_data = field_has_data($field); on lines 1583 and 1911 with $has_data = false;, since this is what prevents the change. This enabled me to change the attachment field from public to private.

(Modifying core files is considered bad practice but this was never intended as a permanent solution.)

After this, I was able to select private. Then I created a test post with an attachment to see how it would be entered in the file_managed table. It was entered as private://test.txt whereas Drupal 6 stored it as public://private/ (where private is the private path relative to the file directory).

Using phpMyAdmin I exported the table file_managed to a local file. Using a text editor I replaced all occurrences of public://private/ with private:// and zipped and saved the file.

Using phpMyAdmin I truncated the table file_managed. You can do this by entering the SQL query: truncate file_managed. Then I imported the saved file. Finally, I restored the file modules/field_ui/field_ui.admin.inc from the original Drupal 7 module.