Drupal 8 change text format for nodes

you can update the text format for all body fields to basic_html via a DB query ( and clear cache to see changes )

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