Should I stop Singapore Math Now, and start using Math U See?

I suppose it would depend exactly why you are switching, but I switched my dd from Math Mammoth to Math U See in the middle of the school year last year with no problems. I did start her all the way at the beginning of Alpha again, and we moved through the first several lessons quickly (maybe spending 2 days or so on each) until we caught up to the fact families where we had left off in MM. This gave her a chance to learn the MUS approach and how the blocks worked, etc, while reviewing the already learned material (and gave me a chance to see what she was already solid on and where we needed more work.)

I went ahead and switched mid-year for a couple reasons: a)I was having to supplement and tweak MM a lot and for me as the teacher it wasn’t sustainable. I didn’t have time to come up with extra games and teaching activities for her to do, but the worksheets alone were not enough for dd. I needed something more teacher friendly. b)Even if we had gone ahead and continued with MM for the rest of the year, I would have switched her to MUS for the following school year, and still probably would have had to start her over in Alpha just because the MUS approach is so different. I felt like it was better to back her up and start her over in the middle of first grade, rather than continue to struggle through something that was not going well for us only to back up and start over at the start of 2nd. This has proven to be a good decision for us.

Hope that helps.

Jen