From Dianne Hackborn:
Things That Cannot Change:
The most obvious and visible of these is the “manifest package name,” the unique name you give to your application in its AndroidManifest.xml. The name uses a Java-language-style naming convention, with Internet domain ownership helping to avoid name collisions. For example, since Google owns the domain “google.com”, the manifest package names of all of our applications should start with “com.google.” It’s important for developers to follow this convention in order to avoid conflicts with other developers.
Once you publish your application under its manifest package name, this is the unique identity of the application forever more. Switching to a different name results in an entirely new application, one that can’t be installed as an update to the existing application.
More on things you cannot change here
Regarding your question on the URL from Google Play, the package defined there is linked to the app’s fully qualified package you have in your AndroidManifest.xml file. More on Google Play’s link formats here.