v25.0 was Summer '12, so the changes you seek would have been covered in the release notes for v26.0 - Winter '13.
Winter '13 Release notes.
The particular section you want from those release notes is:
Event and EventAttendee Object Changes
Important changes have been made to the
EventAttendee API objects. Now any customer or partner can modify
event attendees with the API. These changes might impact existing Apex and API packages.
EventAttendee object is no longer supported.
- There is a new
EventRelation object with fields to manipulate Whos, Whats, and invitees on events. You can modify
invitees on an event.
Note: A Who is a related contact or lead, and a What is a related record.
- Update operations on child events are very limited.
These changes allow you to create and modify event attendees (invitees) through the API. Previously, this was possible through
the UI only. The new API also supports Shared Activities for events so users can relate non-recurring events to as many as 10
contacts. Even if you don’t enable Shared Activities for events for your users, the following event object changes will affect
There is also a useful table in that document you should check.
Generally speaking, the Event is still accessible on the EventRelation via the
The main change for you is that the
AttendeeId has become
RelationId and could be to a User, Contact, or Lead record.