Switching Themes While Preserving Content

We are looking to create WP MU installation. Our goals are:

1) Start with 1 theme, which will be pre-populated with all the content, menus, images, contact forms, etc. defaulting to the admin of each new site installed on MU platform.

2) We would like to add more themes in the future and allow a user to easily switch to a new theme any time they like.

My understanding is that if they switch to a new theme later, it will load all content of that theme, erasing any changes or customization to content they have done.

Question: How do we handle existing content if they switch to a new theme? How can we make it keep and pre-populate the existing content? What other plugins or functionality do I need to make this happen?