Hi morvy,
That will waste a lot of our time. Users should not modify core files, because we can not be sure that with the modification it still works, it also could introduce errors which are not in the original branch of code and drive us crazy trying to find non-existing errors.
If you need to do any modification, I will suggest you create a plugin instead of modifying core files, if the plugin, for some reason, it's not possible, then contact us and we'll suggest you the best way to do it, or modify core files in order to include your modification or allowing you to create a plugin.
If you're talking about the theme, I would STRONGLY recommend, that if you want to modify it, create a new one (or it will be overwritten with each upgrade). In case you create a NEW theme, it's not possible for us to program a batch updater, since the structure of it could be completely different from official theme