Ok, here is code

stuff.java

/*
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package net.rymate.ChangeName;

import java.util.logging.Logger;
import net.minecraft.server.EntityPlayer;
import net.minecraft.server.EntityTracker;
import net.minecraft.server.WorldServer;
import org.bukkit.ChatColor;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.craftbukkit.entity.CraftPlayer;
import org.bukkit.entity.Player;
import org.bukkit.plugin.java.JavaPlugin;

/**
 *
 * @author rymate
 */
public class ChangePlayerName extends JavaPlugin {

    Logger log = Logger.getLogger("Minecraft");

    public void onDisable() {
        log.info(this.getDescription().getMain() + "disabled!");
    }

    public void onEnable() {
        log.info(this.getDescription().getMain() + "enabled!");
    }

    public boolean onCommand(CommandSender sender, Command cmd, String commandLabel, String[] args) {
        if (args[0].equalsIgnoreCase("changename")) {

            if (!(sender instanceof CraftPlayer)) {
                sender.sendMessage("You are not an in-game player!");
                return false;
            }
            EntityPlayer player = (EntityPlayer) ((CraftPlayer) sender).getHandle();
            if (args.length > 1) {
                setPlayerName(player, args[1]);
                sender.sendMessage("Your name has been changed to " + args[1]);
                return true;
            }
            return true;
        }
        return true;
    }

    //thanks to Lahwran for this code :D
    public void setPlayerName(EntityPlayer player, String newname) {
        WorldServer world = (WorldServer) player.world;
        EntityTracker tracker = world.tracker;
        tracker.untrackEntity(player);
        player.name = newname;
        tracker.track(player);
    }

}