Really great news: Manage the #WP7 & #Mango Versions of your Apps separately on the Marketplace!
Maintaining Windows Phone 7.0 and 7.5 Versions of Your Apps Simultaneously
Effective today, developers who have updated their Windows Phone 7.0 applications to Windows Phone 7.5 (OS 7.1, aka Mango) have new options for how they manage updates for both apps. The 7.0 and 7.5 versions of an application share the same catalog details, pricing (including trial and geographic distribution options), ratings & reviews and hidden/live property if in a published state. However, each version of the application can now be updated, unpublished and re-published independently. You can view both versions of the application on the lifecycle tab in App Hub.
It is important to note that there can only be one in-progress update at any time across both versions of an app. The update in progress must be published to Marketplace before another version of the app can be updated. Also, catalog details and pricing changes made as part of a 7.0 app update will affect the 7.5 version of the app and vice versa. Finally, be aware that if you publish a new 7.5 app there is no way to later add a 7.0 version.
More information about maintaining Windows Phone 7.0 and 7.5 versions of your app can be found here.