I never change my whole routine at once. I think my whole routine is just constantly in change.
for major body parts (chest, back, legs) I probably do 4 different exercises, minor I do 3.
One of those exercises is pretty much always a constant, i.e. squats, bench, bent over rows, the other exercises, one of them gets rotated out every 6 weeks (or so, nothing exact).....
for example, my leg routine....
Squat,
Leg press
Leg extension
Leg curls (hams)
I just changed in Hack squats for the leg press last week, and my legs were killing me the next day, I'll do that routine for a while and will probably rotate out leg extensions for something else, and then go back and rotae out hack squats after that