I was recently at a wedding where the bride had made some last-minute changes to her day-of timeline. She was planning on spending more time between the ceremony and reception, and had worked with her photographer to change the arrival time of her wedding party to the cocktail hour and reception. Not a big deal, except that she hadn't informed all of her wedding pros of the change.
The DJ had the updated timeline, but the caterer and bartender were working off of the previous version of her timeline, so the caterer had the food hot and ready to go about 45 minutes before anyone from the family or bridal party was scheduled to even arrive. (Plus, the photos ran long, as often happens, so the arrival was even later than on the revised timeline, causing the caterer to really sweat in keeping the food warm.) Likewise, the bartenders were packed up and gone before the crowd really got going because their contract was about an hour off.
Overall, it all worked out, and the bride and groom plus the guests probably had no idea, but among the people working the wedding, knowing the updated timeline would have made the services they provided much easier.
Bottom Line: Timelines and schedules change - sometimes it can't be helped, and change is not always a bad thing. Just be sure that if you make a change, you let ALL of your involved wedding professionals know about the change. Even if you think the change won't affect them, give them the updated changes anyway - Professionals always prefer to have too many details rather than not enough. If you have a wedding planner, she will usually update everyone involved, but if you are doing it on your own, keep a master list of all of your wedding professionals to make sure you don't leave anyone out during communications.