Distinguishing between iPhone app upgrade and app new install?

There is a situation that I am faced with which seems like it has no solution. Here it is: I have an app on App Store. I didn't write any piece of code for this app keeping the upgrade scenario in mind. For all I could imagine, I never that I would be giving out more versions apart from the one I already have on app store. Now, the situation is that I am require开发者_开发知识库d to give an upgrade of my app and with in the new code, I am to identify if it is a new install of the latest version or if it is an upgrade from the older version. I don't know how to approach this as I have not coded my first version appropriately for upgrades? Anybody willing to enlighten me.

See previous thread here iPhone: How do I detect when an app is launched for the first time? I use this in just your scenario and it's fine to introduce it with the update - it copes with the previous version doing none of this.