Good to know: What happens when I publish a Windows Phone ‘Mango’ application update? (via the WP7 Dev Blog)

How will my update appear across the full scope of all Windows Phone
users?

When you publish a Windows Phone ‘Mango’ update to an application you had
previously published, the following will occur:

• The WP 7.0 version of your application will remain available to all users on
  WP 7.0 devices.
• The WP ‘Mango’ version of your application will be available to all users on
  WP ‘Mango’ devices.
• WP ‘Mango’ users who previously installed your application will receive an
  update notification. This will occur even if they installed your application
  prior to updating to WP ‘Mango’
• WP 7.0 users who have installed your application, but not updated to WP
  ‘Mango,’ will not receive an update notification.
• The WP7.0 rankings & reviews that your application received will attach
  to your new updated WP ‘Mango’ version too.
• The application metadata and screenshots for Windows Phone Marketplace
  submitted with the WP ‘Mango’ version of your application will be shared across
  both the WP 7.0 and WP ‘Mango’ versions of the application; i.e., a single set
  of metadata and screenshots will appear to both WP 7.0 and WP ‘Mango’ users.
• Once you have released the WP ‘Mango’ version of your application, you will
  no longer be able to modify the previously released WP 7.0 version of your app.
  We realize that some of you may worry about this limitation, and so we’re
  actively working on ways to mitigate it. To improve the experience for users,
  it’s our preference not to have “duplicate versions” of your apps in the
  marketplace. We are also going to work hard to encourage our pre-existing users
  to enjoy the free upgrade to Mango as quickly as possible.