Questions about Mac Apps

Hi, I've been lurking on these forums for a while, thought I'd join as I do some software development as a hobby and have some technical questions about the new Mac App Store, sorry if they are obvious or have been answered elsewhere.

1. How do upgrades work if the program is already running - will the App Store close any running instances before installing the new version?

2. To what extent are downloaded apps locked from changes, can a user for instance go into the Package Contents and change an icon, plist or resource image as you can with normal application bundles? I have found this can be useful sometimes for customising things.

Hopefully I can get the answers with more research at developer.apple.com so will post back if I find anything.