we moved house prior to dd starting school, so i can tell you the process we went through if it helps...
dd was starting in the following september, sold house in the feb, moved in the july. prior to selling, we'd applied for a school place in the current lea, got the place, so until we knew everything was definitely going ahead, we far as that was concerned, we just carried on as normal, did the visits etc. for the new area, i filled in the application forms, albeit after the closing date, so we were late applying. we knew dd wouldn't get a school place at that stage, but she did get put on the waiting list (was number 9!).
the new lea, couldn't move dd up the list until we had exchanged contacts, as once we'd done that, dd was the only one on the list actually within catchment. finally, in the july we moved, and after exchanging and moving within a few days, i went to the new lea offices to give them evidence of the new address. only at this point, did i ring the old lea and decline dd's place at our old address (the education officer there had told me to do this in case everything had fallen through and dd ended up with no school place at all). within a week of moving, dd was now top of the waiting list, and a place came up at our preferred school.
we found the waiting list moved a lot, more that you'd think, and we never needed to go to appeal etc. there were no allowances as such for dd moving into the new area, but prior to us moving, every time a place came up, the education officer rang to see how close we were to exchanging. it was hard watching all these school places come and go at the time, as the moving chain had become a nightmare at that point with so many delays, none of which were caused by us.
hope it goes well for you and your ds.