1. When you update Moodle to 2.8 version (we updated from 2.6.1 to 2.8.3), the new field introformat in table mdl_lesson will be added (default value is 0).
This is set in /mod/lesson/db/upgrade.php.
2. If you create (or update it) a new lesson activity, Atto/TinyMCE editors will be showed on the Edit settings page. In the introformat field will set value 1.
3. But when you update old lessons, created in the previous Moodle version, the textarea field will be showed instead Atto/TinyMCE
(because default value of introformat field for old lesson activities is 0, not 1).
4. If you change in the table mdl_lesson in introformat field value from 0 to 1, Atto/TinyMCE will be showed on Edit settings page.

We fix this problem by writing the script which change all values in introformat field from 0 to 1.