Changing Symbols

I bring the solution to all your problems in this simple easy-to-read* script snippet!

#target illustrator
function test(){
    var SymbolSampleScript = {
        displayProps : function (item){
            var msg = [];
            for(var all in item){
                try{
                    msg.push(all + " : " + item[all]);
                } catch(e){
                    msg.push(all + " : " + e);
                }
            }
            return msg.join("\n");
        },
        getPropListFromArray : function (srcArr, prop){
            var arr = [];
            for(var i = 0; i < srcArr.length; i++){
                arr.push(srcArr[prop]);

            };
            return arr;
        },
        SymbolChanger : {
            chooseSymbolDialog : function(){
                var w = new Window('dialog', 'choose symbol');
                var list = w.add("listbox", undefined, SymbolSampleScript.getPropListFromArray(app.activeDocument.symbols, "name"));
                list.size = [150, 200];
                var g_btn = w.add("group");
                var btn_ok = g_btn.add("button", undefined, "Ok");
                var btn_ccl = g_btn.add("button", undefined, "Cancel");
                btn_ok.onClick = function(){
                    if(SymbolSampleScript.SymbolChanger.validate(list)){
                        w.close();
                    } else {
                        return;
                    }
                }
                if(w.show() == 2){
                    return null;
                } else {
                    return {
                        selectedSymbol : list.selection.text
                    };
                }
            },
            validate : function(list){
                if(list.selection != null){
                    return true;
                } else {
                    alert("Please select an item from the listbox.");
                }
                return false;
            }
        },
        main : function(){
            var doc = app.activeDocument;
            var s = doc.selection;
            if(s != null && s.length > 0 && s[0].typename == "SymbolItem"){
                var userInput = this.SymbolChanger.chooseSymbolDialog();
                if(userInput != null){
                    s[0].symbol = doc.symbols.getByName(userInput.selectedSymbol);
                } else {
                    alert("Cancelled");
                }
            } else {
                alert("Please select one symbol instance on an artboard in your document.");
            }
        }
    };
    SymbolSampleScript.main();
};
test();

*relatively-speaking