The game will be completely separate rather than just a simple upgrade. This means you'll pay $60 approx. for the starter pack, as it includes three Avengers, Iron Man, Thor and Black Widow, as the Avengers Play Set is also included. It is a pointless strategy in Disney's place to make it an upgrade, shown in the Street Fighter franchise. Plus, the Avengers in the Starter Pack are not available separately! The base, as far as I know, is the exact same. However, with my own personal experience, the portal could be quite buggy, so hopefully the technology has been improved.
I hope this helped!