BuddyPress.org

As of WPMU 2.8.1 this will work, but with 2.7.1 if you changed an email address, it didn’t get changed in wp_signups, so even if you changed the email address, you still could not reuse the original one you used to sign up. Be careful right now using 2.8.1 as there are several ways that it’s not playing friendly with BuddyPress.

A user-switching plugin is probably the easiest and safest bet, in my opinion.