Hello there,
@Flys:
I believe you can easily achieve this a couple of ways, either you can offer a trial period, and then make it so that they pay for it, or their subscription ends.
Or you can simply offer a link to the registration page for level 0, you can use the following WordPress function in order to output the registration link:
http://codex.wordpress.org/Function_Reference/wp_logout_url
@David:
In the registration page by WordPress’ default, the form there is for level 0 (the free registration link ends with wp-login?action=register).
Without the pro version the only thing you can do is offer a link to the registration page for level 0 (see above). If you have the pro version you can offer a level 0 pro-form, you can find it under Dashboard -› s2Member® -› PayPal® Pro Forms -› Free Registration Forms.
Hope this helps. :)