I did Core for the first 6 or 7 weeks, then switched to flex for a couple weeks because there were several special events that I knew I wouldn't be able to eat at if I was on Core. I will be going back on Core this Fri. It worked pretty well for me. My leader actually switches from Core to Flex every other week. You can change anytime you want as long as you commit the whole week to one plan or the other. I plan to do Core for 4-6 weeks, then flex for 2 weeks for a change of pace so I don't get burned out on Core food. I've lost about 11.2 pounds so far in about 8 weeks. I hope to lose 40 by our trip in April.