To do this you would need to use the MM_Access_Decision SmartTag
Below is an example on how you would show teaser content and full content on the same page based on whether someone is a member or not. In the first snippet we are only showing content to non-members while the second snippet is only visible to members.
Non-members will only see things inside these tags.
Here is your teaser content.
Click here to purchase access: <a href="[MM_Purchase_Link productId='1']">Get Access</a>
Members will only see things inside these tags.
Here is your complete content.
When using these SmartTags, only the content within the SmartTags is protected. Read this article to learn more about protecting the page itself.