My DS moved to a mixed 1st grade/K class around Halloween of his K year. He was with the Kers for art, craft etc but attended the 1st grade class for math and LA and science. He did not have a problem transitioning or making friends. He did have a problem with his old "K friends" labeling his acceleration as "unfair" - he felt sorry that they did not get to do 1st grade stuff with him as well (and felt guilty for having the acceleration). We had to have a lot of talks about how different people have different learning needs and learning paths with him.
If your child is mature, social and adventurous, then there will be no problems in the transition. As others said, the writing requirements were a lot more and the homework load was high too.
DS actually started to complain after a few months that 1st grade seemed like more of the same old stuff. At which point we moved on to private school because the PS had already done what they could for my DS and it was not working.
