Change all article text format to basic html
While you might just need to add/configure the full_html text format, you can update the text format for all body fields to basic_html via a DB query ( and clear cache )

db_update('node__body')
  -&gt;fields(array('body_format' =&gt; 'basic_html'))
  -&gt;condition('bundle', 'article')
  -&gt;execute();

db_update('node_revision__body')
  -&gt;fields(array('body_format' =&gt; 'basic_html'))
  -&gt;condition('bundle', 'article')
  -&gt;execute();


The raw SQL equivalent (e.g for PHPMyAdmin) is:

UPDATE node__body set body_format = 'basic_html' where bundle = 'article';
UPDATE node_revision__body set body_format = 'basic_html' where bundle = 'article';


Backup your database first