There are three choices when setting the Member Registration Options for a Level.
This is located in the Setup > Levels > *Click on Level Name* > Registrations section of WishList Member.
You can set the registration form for the Level to allow New Members Only or Existing Members Only or Both to register.
New Members Only – This means only new members can register for the level. If there is a member who belongs to another level on the site, they won’t have the option to register as an Existing Member.
Existing Members Only – This means only existing members can register for the level. A member must belong to at least one other level on the site in order to be able to register for the level.
Both New and Existing Member – This means new members and existing members can register for the level. This is the most commonly used Member Registration Option.
Note: The Existing Members Only option allows returning Members to login with their existing account so that they can add a new level to their existing account or upgrade to an additional level. This option is required to allow returning members to upgrade or add additional levels to their account.
More info on this topic can be found in the Registration Settings For Membership Levels Knowledge Base article.