We just updated our database from 1.9 to 2.0, which went fairly smoothly as far as the upgrade process. The trouble we're coming into now is with Event Registrations.
We have an event with price sets. While adding new event registrations or editing event registrations in 1.9, we could not edit the price set information on the contact's information page. We could view what price set options the participant selected. Now with 2.0, we can add new registrations appropriately, but we cannot edit the price set options for registrations that happened during 1.9.
Also, when we export participant information, we the Fee Level column does not show the price set options for "1.9" registrations. It does for registrations we just added with 2.0.