and shifting your exercises a bit. do incline bench for a few weeks, then flat bench. also switch between barbell bench press and dumbbell bench press...stuff like that gives you a more rounded routine, and the change in stimulus helps prevent plateus
There are many ways you can "change" your workout. And for many reasons.
But as a rule i'd say its always good to chage your workout.
if your new to weightlifting you may want to change your exercises around, this will keep motivation high and help work parts of the body you may have missed before.
Changing the order of your workout will also give you more energy to work the body parts that you normaly save for last.