What happens to existing users on a BuddyPress site if I import a list of people on Ning?

Hi, there. I'm getting ready to export my Ning network to BuddyPress. There are some people who are not on Ning who would like to register on the BuddyPress site (arlingtonentrepreneurs.org). Will their registration information be over-written when I upload the Ning csv file, or will the Ning information simply be added on to the existing set of users?

I created Arlington Entrepreneurs on Ning several years ago and I have a list of users there. I recently created a new wordpress website for this organization and installed BuddyPress. I eventually want to get the entire user base over to BuddyPress, but don't want to wipe out any new people who may have registered on the site before the Ning-to-BuddyPress migration.

I hope this makes sense! Please let me know how this should proceed.

Best, Margy Rydzynski