My DS did public K and a couple months of 1st before hs'ing. He liked K, but was frustrated and bored in 1st, so he was ready for a change. We slowly mentioned the idea of hs'ing for a month or two before we pulled him out, and the only hesitancy he had was missing his friends. He had only known these kids for a couple months, and wasn't best buddies with anyone, so we were confident it would pass.
He had a just a couple of instances of crying and missing his old class after he'd been home, but only when he was either overtired or something else was stressing him.
Before his last day I took him to pick out business card paper and on the computer we made him cards with his name, our phone # and email for him to pass out to all his 'friends', teachers, etc. That made him feel better, and he did get a couple of calls/emails at first.
We also tried to keep busy going to a lot of hs group events, so he'd quickly meet a lot of kids, and I encourage him to play with neighborhood kids whenvever they are available after school (which he didn't get as much of in school b/c of the hour of homework).
The only other issue in the transition is my expectation that he'd be able to do more independant work/reading than he's been willing to do. It's getting better, but if I had to do it all over again I would try hard to throw out any expectations, and just spend time assessing how to best spend our days.