Yes, you can change the attendees for an event. Go to the Event Attendance Grid in the Events Module. Select the appropriate event from the drop down box at the top of the page. Click on the Edit Icon on the left side of the row for the person being changed. The new dialog box will open to allow you to select another person from the database or choose to add a New Attendee. You can then edit the information for that revised attendee including changing any groups they are assigned to and update answers to the attendee questions.