If your site has several quotas that members may sign up into (for example Fans and Artists), you can let members switch between them later themselves.
Check Allow Quota Changes in ACP > Modules > Profiles > Profiles > Global Config and save.
A Profile Quota field then appears on the member's Profile settings page. They can choose any quota that allows signups.
The field only shows when:
Members can't use it to move into a quota that doesn't allow signups. That's how you keep paid or invitation-only quotas closed.
To charge for a quota (a paid membership), use a subscription module from the Marketplace instead. It moves a profile into the paid quota when the member pays.