He may have already been filled in; I have no idea if any of the new hires know about my past or not. I asked an informed, trusted, employee once, and she said she had never heard anyone talking about it, so who knows.
When those things present themselves, I just introduce myself again, as me, and not say anything. I suppose it depends on how close I knew the person before.
This happened at work a few months ago, when someone I worked with 5 years ago stopped by to say hello. I was really truly busy and did manage to say Hi as I walked past her, but I had no idea if she knew who I was (she knew me pre-transition, obviously) at that point, or if she knew about my transition.
It depends.
I wish I never had to disclose ever. Part of why I want to move far, far away, donkey.
Jay