We used Calvert Math for our oldest child for the first time last year and I really liked it. It helped him "catch up" by explaining new concepts well and giving lots of options for practice with each new skill or concept. Before this, we had used workbooks and simply followed the SOLs for our state for math concepts to cover. Our son has enjoyed the textbook and workbook, too. (Both are workbooks, really.) And both our kids are excited about the new online activities. We only really use the manual when we get to a concept that he's not understanding--to see how to better explain it & some different approaches we can take.
We ordered 1st grade math for our Kindergarten student for this coming school year since she's already adding and subtracting and whizzed through the Calvert pre-K math stuff in a day or two. Practice writing the numbers might actually come in handy for her, but we use Handwriting Without Tears for writing, and it includes number formation practice, so we'll meld the two. She's already done 8 or 9 of the online games for 1st grade. (She's 6). We're still finishing up some of the other pre-K activities, learning games, and craft project though. Even though she's "ahead" academically, we've all really enjoyed the social skills and discussion sections of the lesson manuals, and the stories and verses--which indicate that she's on-target age-wise in these other areas. That's why we went ahead and ordered the full curriculum for her again this year, even though we haven't used it for our son. (Just the Math.)
You can return the K math and swap it for the 1st grade math if you haven't written in the manuals yet. But I'd give a good look through the whole manual first to see if the second half would really be what you need after all.