Installing membership Plugin after having used another membership plugin

I am new here and this is my first visit. I am working on a non profit’s site and they having been using WordPress with a plugin called Register Plus. It doesn’t seem to work right anymore and I’d like to use WPMU Membership. I am not sure, will deleting the old plugin make all the old registered people disappear, or would they still be able to register with the new plugin?

I’m sorry, I’m just not sure how to install Membership and Membership Directory with the old icky plugin still hanging around.

Thanks for any help.