The package name of an app has nothing to do with any class names, it’s just an entry in the manifest. It’s usually the same as the name of one of the packages in the application but it doesn’t have to be. Don’t do a global search and replace (which may mess things up), just edit the package name in the manifest. Note that activity, receiver, and service names are often specified in the manifest relative to the app package name. If this is the case (they start with a dot), you’ll have to change them to absolute class names as well.
If an application gets a “suddenly stopped” error, look at the Logcat output to see what the error was. There’s countless reasons an application can crash so it’s impossible to guess based on the description you gave.