How do I allow free 'Trial' members to subscribe to paid membership?

I am using the Business Portfolio child them with BuddyPress and the membership plugin and 2CO gateway.

I have been testing my site http://sisterhoodofsupport.com and have offered a free Trial membership. Right now I have over 100 members signed up as free Trial members. They all knew that this was a temporary situation and that they will be required to become paid members if they want continued access.

How do I allow them to subscribe as a paid member? When I test the 'sign up' function it will not allow me to put in a user name or e-mail address that is already in use.

Do I have to delete all these members or is there a way to convert them to a paid membership? If I have to delete them is there a way to preserve all the forum and reply entries on the site now under their user names?