I can't seem to find the original thread where switching Processing renderers was asked. I am using some awesome code by harukit for the demonstration.

Basically it uses some code I found on the old processing.org forums to show a second window. Once you do that you can change the render as you desire, how many times, you desire.. and it does not mess with the framerate from some strange reason!.

Here is the code:

Press '1' for P3D
Press '2' for P2D
Press '3' for JAVA2D
Press 'n' for new window (uses P3D by default)

1. 
   import javax.swing.JFrame;
   public PImage a;
   public int mmx;
   public int mmy;
   public JFrame new_window;
   MApplet sketchviewer;
   public int frameWidth =0;//10
   public int frameHeight =0; //25
   public String rmode = "P3D";
   
   int w;
   int h;
   
   void setup(){
     w=255;
     h=255;
     a=createImage(w,h,ARGB);
     size(w,h,P3D);
   
     background(233,23,23);
   }
   
   
   
   void keyPressed(){
     if(key=='n')   loadNewWindow();
     if (key=='r')setup();
   
     if (key=='1') {
       rmode  = "P3D";
       loadNewWindow();
     }
     if (key=='2') {
       rmode  = "P2D";
       loadNewWindow();
     }
     if (key=='3') {
       rmode  = "JAVA2D";
       loadNewWindow();
     }
   
   }
   
   void draw(){
     if (pmouseX!=mouseX && pmouseY!=mouseY)
     {
       this.frame.setTitle(str(mouseX)+ " - " + str(mouseY));
     }
     else
     {
       this.frame.setTitle(str(mmx)+ " - " + str(mmy));
     }
     if (a!=null)background(a);
     mmx= mouseX;
     mmy=mouseY;
     ellipse(mouseX,mouseY,20,20);
   }
   
   
   void loadNewWindow()
   {
   
     // if(new_window == null){ // omitting this allows multiple windows
     new_window = new JFrame();
   
     sketchviewer = new MApplet();
   
     new_window.getContentPane().add(sketchviewer, BorderLayout.CENTER);
   
   
   
     new_window.setVisible(true);
   
     sketchviewer.init();
     //  }
   
   
     new_window.setSize(w + frameWidth, h + frameHeight);
   
   
   }
   
   public class MApplet extends PApplet{
     public PApplet pgx;
   
     //<Insert sketch Code here>
     // Shining Particle by harukit
     // Created with Processing 68 alpha on September 11 , 2004
     // http://www.harukit.com
   
   
     int pNum =4;
     Particle[] p = new Particle[pNum];
     float rr,gg,bb,dis;
     int gain = 5;
     float[] cc = new float[3];
   
     public void setup(){
       if (rmode=="P3D") size(w,h,P3D);
       if (rmode=="P2D") size(w,h,P2D);
       if (rmode=="JAVA2D")size(w,h,JAVA2D);
       loadPixels();
       noStroke();
       background(0);
       for(int i=0;i<3;i++){
         cc[i]=random(40)+random(40)+random(40)+random(40)+random(40);
       }
       for(int i=0;i<pNum;i++){
         p[i] = new Particle(random(width),random(height),random(0.1,0.3));
       }
     }
   
     public void draw(){
   
       mouseX=mmx;
       mouseY=mmy;
       //  for(int i=0;i<pNum;i++){
       //    p[i].update();
       //  }
       for(int y=0;y<height;y++){
         for(int x=0;x<width;x++){
           int pos=y*width+x;
           color col = pixels[pos];
           rr = col >> 16 & 0xff;
           gg = col >> 8 & 0xff;
           bb = col  & 0xff;
           // for(int i=0;i<pNum;i++){
           // dis =dist(p[i].xpos,p[i].ypos,x,y)/2;
           dis = dist(mouseX,mouseY,x,y)/2;
           rr += cc[0]/dis-gain;
           gg += cc[1]/dis-gain;
           bb += cc[2]/dis-gain;
           //  }
           pixels[pos]=color(rr,gg,bb);
           a.pixels[pos]=color(rr,gg,bb);
         }
       }
       if (rmode=="JAVA2D"){
         updatePixels();
       }
     }
   
     public void mousePressed(){
       //  background(0);
       Particle[] p = new Particle[pNum];
     }
   
     public void mouseReleased(){
       for(int i=0;i<3;i++){
         cc[i]=random(40)+random(40)+random(40)+random(40)+random(40);
       }
       // background(0);
       for(int i=0;i<pNum;i++){
         p[i] = new Particle(random(width),random(height),random(0.1,0.3));
       }
     }
   
     class Particle{
       float xpos,ypos,del;
       Particle(float x,float y,float d){
         xpos=x;
         ypos=y;
         del = d;
       }
       void update(){
         xpos += (mouseX-xpos)*del;
         ypos += (mouseY-ypos)*del;
       }
     }
   
   }