Think of someone you used to hang out with in high school. You weren’t best BEST friends but you liked each other and knew each other pretty well.
Then you graduated and went your separate ways and never talked to each other or saw each other after that.
How long can you get away with saying “I know Joe?” At what point does it become “I used to know him?”
I think it depends on how close you once were.
My high school best friend found my name on the Internet and called me a couple weeks ago. We talked for two hours, and it was like the fifteen years that we had barely talked never happened. I don’t think I’ll ever “used to know” her.
On the other hand, I’m sure there’s people I don’t even remember from high school that would fall squarely into the “used to know” catagory.
We had a great 20th high school graduation reunion. I would have to say to the OP, it depends on the person. There were people there I was great friends with in high school, but they changed drastically, so I really didn’t know them anymore. Then there were other people I was casual friends with that didn’t seem to change one bit. So I think you really have to take it on a person by person basis, that is, how much did that person (or YOU for that matter) change since you saw each other last.