What if my reference has changed jobs?
My manager from my last job before law school just informed me that she is leaving the company. Can I still put her as a reference for that job?
Yes. Just keep in touch with her. And the more nice things she's willing to say about you from her position in another context, the more opportunities that more people have to hear good things about you.
As a side note, just as you are asking her to "invest" in your career with a reference, you should offer to do the same for her. Often prospective employers are interested in checking references from subordinates -- it helps to prevent hiring the "Nightmare Boss from Hell" who causes angst and the ever-more-expensive employee turnover.
What about faculty recommenders? This applies as well to faculty references. While GI Generation and Boomer faculty tended to stay put at their institutions, Gen X and Millennial faculty will be mobile. Just because your faculty recommender has left the building doesn't mean that the relationship that you forged is over.
Did you not "forge a relationship" sufficient for the person to write a meaningful letter for you? Just as you have friends around the country and around the world, you can have references from other time zones. Your responsibility is to keep connected. Report in annually and check in on their careers, as well.
To read more about recommendations and recommenders, go to Career Files at "References and Recommendations."