BuddyPress.org

BuddyPress should work on existing installations, but the “buddypress” theme should not be used for regular blogs or enabled for people to use as their blog theme.

Basically the BuddyPress theme stays hidden, and is only used for home bases. Existing members can log in and follow the instructions on the “Create Home Base” tab that they will see once BuddyPress is installed.

As long as you have the “buddypress” theme in the /wp-content/themes/ directory, it should work ok.