My DD moved from K to 1st around Thanksgiving time a couple years ago. Best decision we ever made. It turned out she has ADHD and struggled with focus, but she still has done just fine and is at the top of the higher level class. So now we have the dilemma of the work still being too easy, even with a grade skip. My advice, do it now, because the longer you wait the more resistance you will get, with them claiming that there are too many gaps in learning, too much work to catch up on, too big of a social adjustment, etc. The only thing she really struggled with after being moved was writing, because I never really taught her at home. So she didn't know how to put spaces between words, capitalize, puncuate, etc. The new teacher had to work with her a bit to get her up to speed. This is something you can also work on at home if you haven't already.
At the time I thought the process was ridiculous, with so many assessments needed and hoops to jump through, but now that we are talking about accelerating our younger child, I realize we had it easy and they could have made it much more difficult with DD. We had a principal at the time with a "whatever" attitude and she didn't really care about the hoops.
