diff --git a/lib/unbuild.js b/lib/unbuild.js
index 0e4a8e20..197fb723 100644
--- a/lib/unbuild.js
+++ b/lib/unbuild.js
@@ -19,6 +19,13 @@ var mk = function(path){
 	fs.mkdirSync(path);
 }
 
+var rn = function(path, newPath){
+	path = nodePath.join(dir, path)
+	newPath = nodePath.join(dir, newPath)
+  if(fs.existsSync(newPath)){ return }
+	fs.renameSync(path, newPath);
+}
+
 var between = function(text, start, end){
 	end = end || start;
 	var s = text.indexOf(start);
@@ -65,15 +72,19 @@ var undent = function(code, n){
 
 	var arg = process.argv[2] || 'gun';
 
+	var g;
 	if('gun' === arg){
-		rm('./src');
+		g = 'gun';
+		rn('./src','./old_src');
 		mk('./src');
 		mk('./src/polyfill');
 		mk('./src/adapters');
 	} else {
-		rm('./'+arg);
+		g = arg;
+		rn('./'+arg,'./old_'+arg);
 		mk('./'+arg);
 	}
+	console.log("unbuild:", arg+'.js')
 
 	var f = read(arg+'.js');
 	var code = next(f);
@@ -81,7 +92,7 @@ var undent = function(code, n){
 
 	code = next("/* UNBUILD */");
 	
-	if('gun' === arg){
+	if('gun' === g){
 		write('src/polyfill/unbuild.js', undent(code, 1));
 		arg = '';
 	}
@@ -92,7 +103,20 @@ var undent = function(code, n){
 		var file = path(arg);
 		if(!file){ return }
 		code = code.replace(/\bUSE\(/g, 'require(');
-		write(file, undent(code));
+		code = undent(code);
+		var rcode;
+		try{ rcode = read('old_'+file); } catch(e){}
+		// console.log(rcode);
+		if(rcode != code){
+			console.log("unbuild:","update",file);
+		}
+		write(file, code);
 		recurse();
 	}());
+	if('gun' === g){
+		rm('./old_src');
+	}else{
+		rm('./old_'+g);
+	}
+	
 }());