At nearly every interview, you will be asked if you have any updates for your file. I would wait until this point to tell the Admissions Committee about your new job (or publication, shadowing experience, award, etc). If you email or send a letter now, you risk having nothing to update at the interview. Moreover, you may find in a few weeks or months that you have an additional update, and you do not want to send multiple letters.
Here is my recommended time line. Wait until November or December. At that point, you will (hopefully) have been able to update some interviewers in person. Then, send an email to schools you have not heard from, express your interest in interviewing, and include any updates to your application. If updates occur after you have already interviewed, send an email to your interviewers as well as to the general email address for Admissions and include your updates and also reiterate your interest. Finally, it's nice to have a printed copy of any updates to physically give to your interviewers so that it can be physically added to your file. If you only tell someone, they may forget.