This is more complex than it has been in the past, unfortunately.
Details of the current
In-Year testing process can be found here, and the admission process will remain the same for a few schools next year: http://www.buckscc.gov.uk/bcc/schools/a ... sions.page
However, since all (bar one) of the GS converted to academies, many of them have introduced their own late transfer tests, and a summary of those can be found here: http://www.elevenplusexams.co.uk/school ... -procedure
The new testing process commences once their Admissions Policies take effect in September, and you will need to contact each school you are interested in to apply for testing.
The only schools that might have places in Year 7 after September 2012 are Aylesbury GS, Beaconsfield High, Chesham GS and Sir Henry Floyd because they were "All Offered" at the last count. There might just be gap or two at the other schools, but you would need to ring nearer the time to find that out. (Beware the admissions criteria for Sir William Borlase because they have a requirement for 18 months residence prior to admission.)
Yes, your DD would be allocated an Upper School place before testing, so you need to be careful where you choose to live. Some Upper Schools are great, but others are not so good. You can see which Upper Schools are likely to have places, or be over-subscribed, here: http://www.elevenplusexams.co.uk/school ... n-profiles
If you tell us which area of Bucks you are considering we may be able to give you more specific advice, but I am afraid it is "all change" in these parts at present and your DD will be a guinea pig in the new system.