A simple symbol swapping Adobe Illustrator script made to reproduce the functionality of the Replace dropdown for symbols.

symbol-swap.jsx

// define the document width (in points)
var width = 100;

// define the document height (in points)
var height = 100;

// create a new document
var docPreset = new DocumentPreset();
var presets = app.startupPresetsList;
var preset = presets[0]; // Default: Art & Illustration (Doesn't really matter but addDoocument needs it)  
docPreset.width = width;
docPreset.height = height;
docPreset.colorMode = DocumentColorSpace.RGB;
var doc = app.documents.addDocument(preset, docPreset, false);

// shift origin
doc.rulerOrigin = [0,height];

// add to existing layer
var layer = doc.layers[0];

// draw a square (rect) w/ fill
var square1 = rect(layer, 0, 0, 10, 10);
square1.filled = true;
square1.stroked = false;
square1.fillColor = makeColorRGB(0,0,255);

// draw a square (rect) w/ fill
var square2 = rect(layer, 0, 0, 10, 10);
square2.filled = true;
square2.stroked = false;
square2.fillColor = makeColorRGB(255,0,0);

// make a symbol for the squares
// set their names
var newSymbol1 = doc.symbols.add(square1);
newSymbol1.name = "square1";
var newSymbol2 = doc.symbols.add(square2);
newSymbol2.name = "square2";

square1.remove();
square2.remove();

// make instance of square1 symbol
var mySymbolItem1 = doc.symbolItems.add(newSymbol1);
// set its position
mySymbolItem1.position=Array((20),(-20));

// make instance of square2 symbol
var mySymbolItem2 = doc.symbolItems.add(newSymbol2);

// make instance of square1 symbol to be replaced by instance of square2
var mySymbolItem3 = doc.symbolItems.add(newSymbol1);
mySymbolItem3.position=Array((40),(-20)); // set position for instance

// update position of this instance to that of mySymbolItem3
mySymbolItem2.position=Array((mySymbolItem3.position[0]),(mySymbolItem3.position[1]));

// remove the temp symbol instance
mySymbolItem3.remove();


// make color utility function
function makeColorRGB(r,g,b){
  var c = new RGBColor();
  c.red   = r;
  c.green = g;
  c.blue  = b;
  return c;
}