First, the original AH board and counters are sufficient to play the L2 version. The only changes are for ease of play and aesthetics. The main changes are in the rules and there are really not many of those. I will summarize some of these changes here:
1. Neither naval nor air bombardments advance the impulse marker.
2. The weather cannot change on a naval or air bombardment impulse.
3. The Advantage is no longer worth 1 VP.
4. It is possible to have two corps artillery in one Area, but there are some restrictions on their use.
5. If there are fresh German Coastal Artillery that can affect an allied landing, it is no longer necessary to predesignate all units that will be landing ahead of time.
Here is a link to the L2 changes from official site of the WBC Tournament: