Friendship in Kingdom Hearts
"Make new friends, but keep the old." It's a song we've known since Kindergarten but rarely do we put it into practice for various reasons. It's all too common for friends to grow apart or have a falling out. If you have been fortunate to have the same close friends since your school days, you should treasure those relationships even more than you already do. This is no Yu-Gi-Oh! friendship speech ala Tea Gardner - this is a real-life dissection of friendships using Kingdom Hearts as an example once again.
In Kingdom Hearts I, Riku gets turned against Sora because he thinks Sora has forgotten about him and Kairi and replaced them with Donald and Goofy. Donald doesn't help matters by saying that Riku can't travel with them, as he interprets Mickey's orders to stick by the Keyblade in the strictest sense to mean that they alone have to accompany him (which also leads to them abandoning Sora for Riku when Riku takes the Keyblade away from him). Although Donald and Goofy ultimately side with Sora over the possessed Riku before the Keyblade returns to him, we learn that some friendships can be fickle. Not Sora, though - he never stops being friends with anyone and travels the various worlds befriending everyone (seriously, he could just as easily be the protagonist of Stick of Truth for all the friends he makes if the Kingdom Hearts universe had Facebook). Riku and Sora's friendship survives, and Riku befriends Mickey and Ansem the Wise.
Unfortunately, not everyone gets a happy ending like our main protagonists. We see in Birth By Sleep that Isa and Lea were best friends who wanted to become Ansem's apprentices but were denied. They also joined Organization XIII together, but then something changed. Despite the two plotting to overthrow the Organization (with Axel doing the dirty work), Saix became a vessel of Xehanort like Xigbar had, and Axel notices the change; suddenly Saix has become distant and much better at lying. Axel then gains a new best friend, Roxas. Despite his facade of not caring, Saix becomes jealous and hostile towards Roxas. This happens in real life too because you can't make your friends be friends with one another. Either through jealousy or just plain not getting along, you can't make everyone happy, and that makes you very unhappy. Axel is forced to make a choice between a "real" friendship and a "fake" one (between Roxas the Nobody and Xion the puppet, but Saix is also implying it to mean between him and Roxas).
Your choice of friends and ability to keep them say a lot about you as a person, even if it's not your fault that something in the relationship has changed. For Axel, Roxas represents the light and Saix the darkness. While Axel continues to support Saix's ambitions, he too leaves the Organization to follow Roxas. After they have returned to normal in Dream Drop Distance, Lea is dismayed to find that Isa is still trapped in Xehanort's Organization, and Isa doesn't look like he wants to fight his friend anymore. While this is an extreme example, it's still important not to give up on your friends if they meant anything to you at all. We'll see if Lea can save Isa from Xehanort in Kingdom Hearts III (and if everyone else can save Braig and Terra).
In conclusion, making new friends does not mean you have to discard your old ones, as the song says. True friendship lasts a lifetime (or several if you believe in reincarnation). These relationships are also prone to change, either internally or through manipulation by outside forces. However, as this series shows, sometimes you have to fight for your friends.