I don’t do MEP, but I did do Singapore in the early years with a few of my kids. You are right that it moves quickly.
What we did (instead of beginning a new curriculum mid-year) is to park it where the child is feeling comfortable and begin a more activity oriented math (games, manipulatives, etc…) there. We might also do a worksheet page once or twice a week.
My kids always seemed to go through learning spurts in math, and generally within a month or two of alternating paper practice and manipulatives, they were ready to move on to the next concept. Just my .02 :).