Something like that happened to me at my job recently. I recognized someone from my high school one day was there. My 1st reaction was kinda negative, while I never really had a problem with this particular person his best friend back in the day I really really despised. I thought he was just a nasty POS kid.
But that was over 20 years ago and if the situation presented itself again I think I would at the very least introduce myself. That would be on him how he reacts.