Moodle does not throw any events whenever a user profile field is updated. That goes for both 1.9 and 2.
Within Moodle user profile fields are updated on the edit user profile page which is located in /user/editadvanced.php file.
If you look at that file you will see that after something is updated regarding user it calls user_update method of the authentication plugin that is assigned to that user.
So assuming that the only place where you can change user profile is that page the best way to handle it is by writing your own authentication plugin and use that to handle most of your users. So when user profile is updated update_user method of your authentication plugin will be called and you can put there whatever you want.
On the other hand if user profile fields can be updated by some arbitrary way directly in the database than you are mostly out of luck.