Navigate to the Referees Tab on the left menu and then click Assigning. Under Assigning, navigate to "Match List". This is the area where you would want to review matches that have been rescheduled or changed by the event. Reviews would be needed if there were fields, time, or the venue changed or something else of that nature that would impact the match. 

Use the search filter titled "Needs Official Review". In the dropdown select "true" and then click search.

This will bring up the matches and highlight them in red. In the "referees" column it will also state "Review" in red lettering. Click assign to view the game details.

On the game details page, under the referees, you will see a section titled "Previous Match Assignments". The referees will be highlighted green if they are still available or red if they are not available for this match. Click "Restore Available Previous Assignments" to restore old assignments. You can "Clear Unavailable Previous Assignments" to clear the referees that are no longer available.