In both OJS 2.x and in OJS 3.x the username is simply stored in the users table, under the username column. All references to the user by foreign key are handled by the table’s primary key, user_id. Changing the username should not break database referential integrity.
There is one outlier I’m aware of in the UI: in both 2.x and 3.x, OJS stores uploaded files by username, rather than by userid. In 3.x there is no facility for a user to manage their uploaded files, so change the username will not affect functionality. In 2.x, there is in link in the upload dialog which allows a user to manage their existing uploads. This would break when the username is changed.