you'll be making new friends. but as long as once you meet them, you introduce them to your other friends. so this way, your old friends will be friends with your new friends. and vice versa. make sure they do that same thing is what i mean. then try not to abandon them. i made that mistake in a way. they slipped away from me and now i have to keep trying to get them back. i mean it's easy, but it's not the same as it used to be.
to your second question, STAY IN TOUCH, STAY IN TOUCH, STAY IN TOUCH ! lol that's all i can tell you. when you hangout with your friends, MAKE SURE SHE COMES TOO! it's simple really.
i think you're over thinking this actually. lol. but i thought of the same things. it happens to everyone. just MAKE SURE that
first) DO NOT ditch them for new friends. unless they become everyone's friend too. then it's the same as hangingout with one of your good friends. this way they don't get all mad and talk behind your back because you're hangingout with someone that they don't really like.
next) stay in touch with them always. plan a movie night every friday at eachothers houses and don't invite new people. then you can all hangout and everything.
and with the guy thing, it's the same thing that applies to the girls. hangout with them and like, talk to them online all the time. drop them a call. idk lol. same things basically.
hope that helped(:
good luck !
and remember, don't over think these kind of things. ;]