Here in Birmingham, we did start with the idea of starting the new dataset early. This would be the neatest solution as it actually reflects the reality of what was happening in a school. If, indeed, it is possible, this would be good news all round.
However, we dismissed this solution because of complications with stats in general and Census in particular. I can't remember all of the details but I'm sure that there were problems with attendance and exclusions. In particular, as I understand it, the Autumn census reports on all exclusions from the Summer term and an early end to the dataset might affect this number.
There maybe other subtle issues. For instance, if there's a sixth form, I'm not sure how QAMS will be affected.