In out experience, our DS who was able to easily make friends had no problems making friends after his grade skip, and again after transferring to an accelerated program another year ahead.
As for academics, after one grade skip in our local school, DS was still getting the highest test scores in the class. He had probably the worst handwriting in the class, but that wasn't a huge change as it was poor before the skip. We did talk to him about how he would probably be one of the slower kids in gym, but as he wasn't all that coordinated at that age anyway, he said he didn't mind since he was already pretty bad in gym.
We got what we really wanted for our kiddo when we transferred to the accelerated HG program -- he was finally appropriately challenged and not always top of the class in everything. The teacher pointed out that since the program was a big change for most kids since they were finally getting challenged, it was common to go through a phase where there may be tears and complaints of things being too hard, but really the kids were just experiencing how to learn in school for the first time. We did discuss this possibility with our son, to help him know what to expect.
We also told our son that we were doing our best to find him an appropriate school situation. I think it helps kids a lot to know that the parent is on their side, even if things don't work out.
How the transition goes also depends somewhat on your kid's personality. My kiddo is pretty laid back and doesn't like standing out. He's also a chameleon and can fit pretty well in any group. Things went fine for him.
