Classipress is a theme and so it will change the look of your site if you use it instead. However, you may opt for installing a wordpress/classipress in subdomain so that your existing site will not be touched.
Please help our moderating team work more efficiently by not sending us support questions via PM. You can read more about how AppThemes support works
here. However, you can send a PM to follow up and remind me if I missed your support request/thread.
Thank you and have a nice day.