It is a good idea to change your workout plans every so often to get out of the rut of doing the same workouts all the time.
By changing the program it keeps things interesting and allows your body to continue developing.
However, if you follow the same workout for a prolonged period of time it is very likely that one of two things could happen:
1. You might get bored and lose interest in training due to the monotonous workouts.
2. You hit a plateau with your progress