Hi,
the ones which were composed with the drag and drop editor, even if they now appear as not drag and drop can be recovered in this way: copy their HTML with the raw HTML editor. Create a new drag and drop newsletter, proceed to the targeting phase, switch to the raw HTML editor and paste the original HTML you previously copied. Save and go back to the list, it should show now as a blocks newsletter and editable.
It's a long process I know.
Regards, Carola.