Change Scenes in Visual Midlet

Question

I am using VisualMIDlet to make a RPG game in Java Me (sun_java_me_sdk-3_0-win) and I don't know how to change the Scene when game is running.

In the init() before the "real" game runs I have a "map selector" in the main menu:

public class LienzoDelJuego extends GameCanvas implements Runnable {
public LienzoDelJuego(VisualMIDlet unmenu, int mapa) {
        super(true);
        this.menuPrincipal = unmenu;
        this.nummapa = mapa;
        this.pausa = false;
        d = menuPrincipal.getDisplay();
        try {
            this.setFullScreenMode(true);
            this.init();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
    }

and the init():

private void init() throws IOException {
        this.timer = new Timer();
        this.gameDesign = new Diseny();
        this.lm = new LayerManager();
        g = getGraphics();
        ...
switch (nummapa) {
            case 0:
                this.tlTown = this.gameDesign.getNULL();
                this.tlBlock = this.gameDesign.getCasa2();
                this.tlWater = this.gameDesign.getNULL();
                this.tlBase = this.gameDesign.getCasa1();

                gameDesign.updateLayerManagerForCasaW(lm);

                break;
            case 1:
                this.tlTown = this.gameDesign.getNULL();
                this.tlBlock = this.gameDesign.getTown2();
                this.tlWater = this.gameDesign.getNULL();
                this.tlBase = this.gameDesign.getTown1();

                gameDesign.updateLayerManagerForTown(lm);

                break;
}

and in the run() {} I have a switch too with "nummapa" (the name of the scene) to act different if the number is 1 or 2. The problem is when I try to change the scene or map in the run() code, I really don't know how to do it, I tried this but does not show the textures and sprites of the other map:

        else if (spriteW.getY() > 190 && spriteW.getX() > 250 && spriteW.getX() < 270 && nummapa == 0) {
            nummapa = 1;
            adjustViewport(0, 0);
        }

I tried setting some objects to null, "gc" them and recalling but the game freezes. Which is the property way to do that? Thanks!