Login to http://www.yourwebsite.com/wp-admin to get to the site admin dashboard.
Now, refer to the following images when wishing to convert a "Trial" (time limited) member to a "Client" (lifetime) member. Follow steps 1 thru 9b to convert existing trial members. Repeat this process for each user conversion.
Step 10 onwards is to be applied to each protected content item/post to ensure both trial and client members can see the content.
Trial user to client conversion complete.
When creating new "members only" content please also remember to allow both trial and client members access to the content as shown below in step 10: