Just to put the contrary view here.
We moved our son this school year. He had taken forever to settle at the school he was attending, but had finally made one good friend. His class teacher was awful, and he was getting no help at all from the school to deal with some fairly major issues with writing and spelling (due to a long delay in replacing the SENCO after the last one left). I was told when I asked why he was not getting any help, that this was because on average he was coping. It was playing merry havoc with his confidence and his motivation.
We went to an open day at the new school, then interviewed the headmaster being crystal clear about the issues (he was a little surprised but I think appreciated our honesty). On his advice we decided to move DS at the end of year 5, rather than waiting for year 7, which was our original intention. DS was underwhelmed by this speeding up of the agenda.
DS had then two days assessment at the school in February, plus a familiarisation day in early June plus a morning meeting his new class teacher just before the end of term. They actually offered him a place immediately after the February assessment but after discussion with ds he finished the year at the old school and moved at the start of p6.
He has settled in at the new school as if he had been there always. He has made three new friends, and is making much better progress with his school work. We are so glad we bit the bullet and moved him when we did.
The key to a good move seems to have been finding the right school for ds, making sure that the school knew what to expect, plenty of familiarisation time for ds and giving him a little bit of say in the timing of the move.
Oh, and he still sees the friend from the last school regularly.