We're currently doing this - DS got our second choice school last year. This year, DD was offered a place at our first choice, and DS is now number 1 on the list to move across by virtue of being a sibling. We have been told he should get a place, as someone has given notice, but they don't offer the places until later in the term - for September. All a bit nervewracking!
If the school wasn't so massively oversubscribed, I would have just contacted the school we wanted him to go to, and if they had had a place, they would have moved him over immediately.
Good luck!