Can you use any application whilst it is being updated Is it possible to use any application while it is being updated from the Mac App Store?
Under System Preferences / Software Update you can choose Automatically check for updates which will enable the Download newly available updates in the background option. 

This is quite self-explanatory. It will just download the update, letting you use the application while the download is active, and when the download is complete, you'll be prompted, through Notification Center, to Install the update.