nice topics you're starting
um...yeah~ this definitely happened to me when I transitioned from different stages of schooling (elementary to highschool to university) Every time, the next school I went to, not a lot of my old school mates went to so I lost contact with people and met new friends.
I still see some of my Highschool friends on and off and the occasional elementary school friend, it's cool to get into contact with people you haven't seen in a long time. But honestly, I wouldn't feel bad about not keeping contact (unless your friend tried really hard to contact you but you just kept ignoring him/her). Changes happen all the time, friends come in and out of your life, it simply happens.
Just because you don't talk to someone for a long time doesn't mean you're not good friends with them. Two of the best friends in my life I haven't talked to in the past month, but we write letters, talk online, etc. When we DO meet up tho, it's fun We just kinda pick up where we left off, they're like family to me, being apart doesn't mean loving them any less.
But yeah contact your friends once in a while. They'll love the surprise and you'll love the company. Ask your other friends about a lost contact maybe they can help. I've had new friends who actually knew my old friends which kinda reconnected old friendships again too ^^ surprise to all of us! XD