For backward compatibility with old third-party modules, we will add a new function called grades_grab_grades() to admin/cron.php to search all mod/xxx/lib.php files for functions named xxx_grades().

These legacy functions will be called to extract all the grades for all the activities in each course. Once the data is extracted, the trigger_event() function can be called to initiate an event as usual and copy/upgrade the data in the gradebook tables.

Think hard about some way to prevent doing this same work over and over ....