How can I change the zenphoto directory name to something else without having to do a complete reinstallation.

You don't have to reinstall, you have to re-run setup to fix the htaccess rewrite base or do it manually. This has really been discussed to death and just the last week if I recall right...

I did a search but unfortunately I did not find the answer. Thanks for responding tho. How do I run the set up files if the original files were deleted? Will I have to reenter the database credentials if I rerun the setup again?

No, just copy the setup.php file and the setup directory to your server then go to http://www.yoursite/zp-core/setup.php

We have now a troubleshooting entry about this:
http://www.zenphoto.org/news/how-to-rename-the-folder-zenphoto-is-installed-in-