I am in need to create a build having the modified and newly added files in an application. Say we have two folders "current" and "old". i am in need to take the files from folder current(modified and new files).
If all you are doing is moving files, look at the copy task. It has an optional overwrite attribute, which defaults to false. So Ant only copies the files which have been created or modified since you last ran the copy task.
Most Ant tasks behave in this way, only acting on newer resources unless you explicitly tell the target to overwrite everything.