You can't, since the PHP module handles the "application/x-httpd-php" mime type, and if you have two modules loaded, both of them would handle the same mime type. Probably the first to load wins. Module loading is done once upon staring the Apache instance, so you can't configure VirtualHosts to have an other module for PHP loaded.
You can do it with configuring your web server to use CGI, and running the PHP code as CGI script, you can have different interpreters for different type of PHP files. For this, you have to name the "CGI" PHP files to something else so that the original handler wouldn't recognize them.
However, you should really consider upgrading your server. A webserver from 2008 is a relic, and the PHP version you use probably has tons of bugs / security holes fixed long ago. If the old websites needs to be changed, so be it. This price for security doesn't seem too high.