Change default page (redirect)

I would like to change my default page of wordpress from index.php to article.php. I renamed the file index.php to article.php and now if I go to article.php it works fine.

The problem is that there are many links already to the blog using /?p=XXXX and that obviously goes to the index page. How can I set my site to redirect any /?p=xxxx to /article.php?p=xxx

Thanks