Its good to change up the routine once in a while to avoid plateaus or when a plateau develops itself. Its a good idea to change it up on a progressive scale, no problems.
A change in a routine could be (for example only): say you bench press 100lbs, for 9 reps, but just can muster that 10th rep. In between sets your currently doing a 2m rest period. One method of change is after the 9th rep, and trying but failing that 10th rep, is to bar the weight, rest say 15 seconds, and pump out as many as your can (this in itself just provided a change in stimulous not normally provided). Then you would rest your 2m before the next set. This is just an example.
Cyclists may peform the same type of exercise, but the intensity and duration are normally not the same, in most cases.
A very common mistake made by beginning weight lifters is staying with their original routines for too long. They want to achieve the same gains they initially experienced in the first six weeks or so of their training and resist change because they feel they really found something that works for them.
The truth is that most decent weight training routines combined with appropriate diets and sufficient rest will produce impressive gains for a while (at first especially). But then the body becomes accustomed to these stresses and the gains may begin to slow.
If one is keeping a training journal (and they should as progression is a key factor), this essential journal can tell you many things: your progress from one work out to the next (and individual exercises), and whether your progress is occurring good and/or slowing. Its this sort of detail, than can tell you if you need to change an exercise (apply a different stimulous), and in effect tell you if you need to change things up. In addition, if one is keeping a diet journal per day along with the training journal, one can look in the diet journal per day and compare results in the training journal and in effect determine if its something in the diet or training and/or both is slowing progress (not to mention personal intensity application to the exercise and overal routine, as there are many factors that can hamper progress).
I believe its a good idea to change things up dependent on several specifications relative to the individual person. Change in fact is sometimes a good idea.
