worriedmum
I don't know the whys and wherefores for your dc, but I moved school May half term of year 10, due to my dad being in the forces and being moved from Germany to England at that time. My parents went around all the schools in the area, and were able to get me in a place that most closely matched the exam boards I was taking. I think I had to do a little extra to cover work I had missed, but was lucky enough that I was mostly in line, albeit with different set texts in English (Henry V instead of Macbeth/Tess of the D'Urbervilles not Lord of the Flies). I would say though, I was top sets across the board, very capable, and used to moving. It wasn't difficult for me to slip into a(nother) new school and pick up as best I could. Depending on your child (ability, personality etc) it may be different for them. I would say get the prospectuses of the schools in the area you are moving, visit them, speak to the Heads/Deputies/Head of year(s), get a good feeling for the schools before you decide where you want them. If it means missing a week or so, it will be less harmful than moving them into the wrong school/wrong year group.
Its a while ago now, but I am willing to answer any questions as best I can. Oh, and I walked out of school with 3 A, 3 B and 3 C, which at the time was a good set of results (A* had only just been introduced)