For me it depends on the site and the participants. I change the exercise for one body part at a time over the course of year. As an example, in a boot camp situation I would change a partner row (two ropes tug of war style) to a TRX row. The next week I change another body part/muscle group exercise like a push up to a high tension tubing chest press one arm at a time (really engages the core as well as the chest). Each week one muscle/muscle group (or two opposing groups) get changed until I have changed all of the sets one time through. Then the process might start over or maybe we start a brand new routine and restart the one exercise change at a time. I try to make enough of the class new enough to notice, but not so new that the benefits of any one exercise are not given a chance to be gained.